50C 7i?0 * i r\ HARVARD UNIVERSITY LIBRARY OF THE Museum of Comparative Zoology mi. cis?. zm i&mi mmt mmm GO*' VOL. 1 2 1 RANS ACTIONS OF THE ociety for British Entomology World List abbreviation: Trans. Soc. Brit. Ent. EDITED BY E. J. POPHAM, D.Sc., Ph.D., A.R.C.S., F.R.E.S. WITH THE ASSISTANCE OF W. A. F. BALFOUR-BROWNE, M.A., F.R.S.E., F.L.S., F.Z.S., F.R.E.S., F.S.B.E. W. D. HINCKS, D.Sc., F.R.E.S. B. M. HOBBY, M.A., D.Phil., F.R.E.S. G. J. KERRICH, M.A., F.L.S., F.R.E.S. O. W. RICHARDS, M.A., D.Sc., F.R.E.S., F.S.B.E. W. H. T. TAMS Date of Publication: 27th March, 1957 Copies may be purchased from the Secretary: Department of Zoology, The University, Manchester, 13 Price 12s. 6d. post free rp^xn SOCIETY FOR BRITISH ENTOMOLOGY OFFICERS AND COUNCIL, 1956-7 President: G. S. KLOET, F.R.E.S., F.Z.S. Vice-Presidents: W. H. THORPE, M.A., Sc.D., F.R.E.S., F.R.S. H. E. HINTON, Ph.D., B.Sc., F.R.E.S. S. C. S. BROWN, L.D.S., R.C.S. Hon. Secretary: J. G. BLOWER, M.Sc., F.R.E.S. Department of Zoology, The University, Manchester, 13 (Ardwick 3333) Hon. Treasurer: Hon. Editor: H. M. MICHAELIS E. J. POPHAM, D.Sc., Ph.D., A.R.C.S., F.R.E.S. 10 Didsbury Park, Manchester, 20 Department of Zoology, The University, Manchester, 13 Other Members of Council: A. BASKER, M.D. E. LEWIS, F.R.E.S. E. HALL, B.Sc., A.R.C.S. A. A. LISNEY, M.A., M.D., D.P.H., F.R.E.S. D. HINCKS, D.Sc., F.R.E.S. H. M. RUSSELL J. KERRICH, M.A., F.L.S., F.R.E.S. A. H. TURNER, F.Z.S., F.R.E.S N. KIDD, F.R.E.S. W. A. WILSON All official correspondence should be sent to the Secretary (List of Publications for Sale (all prices are post free) All dated 1951 or earlier ( Journals excepted ) are subject to a surcharge of 50% JOURNAL Vol. i (Parts 3, 5, 6, 7, 8 and 9) and from Vol. 2, Part 2 onwards, are available. TRANSACTIONS GENERAL A New Chapter in Zoological Nom¬ enclature : The Reforms instituted by the Thirteenth International Congress of Zoology, Paris, July. 1948. By F. Hemming, 1950. 8 pp., 15. 6 d. The Problem of stability in Specific Nomenclature, with special REFERENCE TO CASES WHERE TYPE MATERIAL IS NO LONGER IN EXISTENCE. By F. Hemming, 1951. 16 pp., 25. od. A Preliminary Enquiry into the Influence of Solar Radiation on Insect Environment, with Special Reference to the Relation between Pest Epidemics and Fluctuation in Solar Radiation. By W. B. R. Laidlaw, 1951. 64 pp., 6 figs., 7 s. 6d. Some adaptations of insects to en¬ vironments THAT ARE ALTERNATELY DRY AND FLOODED, WITH SOME NOTES ON THE HABITS OF THE STRATIOMYIDAE. By H. E. Hinton, 1953. 20 pp., 3 figs., 55. od. ENTOMOLOGICAL FAUNA OF THE NEW FOREST SERIES Introduction by J. Cowley, and Part i, Odonata, by Lt.-Col. F. C. Fraser, 1950. 12 pp., 15. 6 d. Part 2, Neuroptera, by Lt.-Col. F. C. Fraser, 1951. 12 pp., 15. 6d. EPHEMEROPTERA Descriptions of some Nymphs of the British Species of the Genus Baetis. By T. T. Macan, 1950. 24 pp., 6 figs., 2 tables, 35. od. HEMIPTERA-HOMOPTERA Revision of the British Species of Cixius Latr., including the Description of a New Species from Scotland. By W. E. China, 1942. 32 pp., 12 figs., 25. 9 d. New and little-known Species of British Typhlocybidae with Keys to the Genera Typhloceba, Erythro- neura , Dikraneura, Notus , Empoasca and Alebra. By W. E. China, 1943. 43 PP-, 14 figs., 45- o d. ( Continued on inside back cover) 1 1 ti L _ il_ ^ u _ _ J ,il Fig. i. Climatic data. Top: Mean air temperature per week at Ambleside. Middle: Stream temperature in degree hours above 50 C. per week at Outgate. Bottom: Daily rainfall at Ambleside. 130 [March as in lower reaches, been washed away because there was never a great weight of water. The remaining stations were in the two tributaries Sykeside Beck and Outgate Beck. In Sykeside Beck at the collecting station (Ss in the tables), the current was moderately swift, relative to other parts of the beck, and the bottom of stones of various sizes left behind after the fine material of a glacial deposit had been washed away. Outgate Beck flowed more slowly in an artificial channel with stones of various sizes. Not far above the collecting station (Og in the tables) it originated in a moss now kept dry enough for pasture by a drain right through it. Methods Collecting was done mainly with a net that had a coarse mesh of 20 threads to the inch. This was used for five or ten minutes to give results that were to some extent comparable. Later, collections were also made with a phyto¬ plankton net (160 threads to the inch), because it was found that an astonishingly high proportion of tiny nymphs passed through the coarser mesh. Some collecting was done with a shovel sampler, by means of which one-twentieth of a square metre of substratum could be removed and tipped into a strong solution of calcium chloride, where the stones and finer particles went to the bottom and the organisms floated to the top. The work covered three years and each station was visited once a month, except in the first year, when the necessity for such regular sampling was not appreciated. The effective start was at the beginning of 1950 except at station 4 which was not included till April, 1951. Climate Late in 1950 a thermograph was set up in Outgate Beck, but unfortunately it proved to have a defective clock and continuous records have not been obtained until after the end of the present investigations when a new clock was fitted. Such results as were obtained, calculated as total degree hours above 50 C. per week, are shown in fig. 1. 1951 has been superimposed as a dotted line on 1952. It is difficult to gain a true picture of the temperature of a stream from readings taken at one place, and the site chosen, or rather the site forced upon me by the circumstance that nowhere else was there safety and shelter for the recording part of the instrument, was just below an underground stretch. During a hot dry spell when the flow was slight, evaporation in this tunnel cooled the water several degrees. The water temperature record being incomplete, the air temperature has been included in fig. 1. The readings are those kept by the Lakes Urban District Council at Ambleside, a place some three miles distant. The histo¬ grams show the average of the maximum and minimum daily temperatures for a week. 1950 has been superimposed as a dotted line on both later years. It is noteworthy that, from the first week in February till mid-May, 1951 was almost consistently colder than 1950. 1952 was colder than 1950 fairly consistently till the first week in April and then warmer in all but one of the next seven weeks. Along the bottom of each year is shown the rainfall, also measured at Ambleside. This place is nearer the mountains and therefore receives more rain per annum than Outgate, some 10 inches (254 mm.) on the average. Occasionally there are marked differences in distribution, but, on the whole, 1957] I3I the pattern in the two places is the same. Rainfall was rather similar in the three years when the studies were made. Fig. i includes no information about 1949 though some mention of the climate of that year must be made, as it wTas during it that most of the nymphs caught in the first part of 1950 hatched. The summer was exceptionally fine and during the five months May, June, July, August and September only 13.54 inches of rain fell. In the last three months of the year 9.40, 13.28 and 14.43 inches fell respectively. The period June 11-July 12 inclusive was without rain except on one day when .03 inches fell and Sykeside Reck was dry in mid-July. The following years had wetter summers than this. 1951 was the driest with little rain from mid- April to July though the longest period without rain was shorter than in other years. Sykeside Beck was dry in June. 1950 was similar in that May and June were the dry months of the year but the period of slight rainfall was shorter. In May, 1952, there were nineteen consecutive days that were rainless, except for one, when .01 inches fell. This was a longer continuous dry period than in either of the previous years, but it came immediately after a wet spell and Sykeside Beck did not dry up. There was another rainless period towards the end of the year, and this was the only time, during the three years, that the level of the beck was recorded as low in the winter. Rain was fairly evenly distributed through the rest of the year. Life Histories and Migrations Rhithrogena semicolorata (Curtis) The total number of specimens collected per month from all the stations except station 4 is shown in fig. 2, which, however, omits the early part of 1950, because collecting was not regular then. In both the years shown, and in 1950, there is a fair or large population in May and a small one in June. The collecting was done in the third week in the month and therefore it is evident that there is a big emergence of adults in late May and the early part of June. In 1952 emerging adults were trapped and the dots in fig. 2 show the numbers caught. By July there are very few nymphs left and the new generation appears in August, September or October. Clearly this species has one generation per year, emerges in the early part of the summer, and grows throughout the winter. That this is the life-history of R. semicolorata has already been shown by Pentelow in the River Tees (Butcher, Longwell and Pentelow, 1937, PP- r42> 144) and Harker (1952), but, when close comparison is made with Harker’s findings, three notable differences are seen. First Harker’s specimens reached a greater size, the largest being over 14 mm. long; in Ford Wood Beck in May, 1950, 72% of the specimens were 8-10 mm. long and only 15% over 10 mm. long, the largest being less than 12 mm. long. In the other two years the proportion of large specimens was less. Secondly the emergence of Harker’s specimens went on longer (in a letter Dr. Harker writes that emergence began in May and that most adults came out in June and July), and thirdly the new generation appeared in July, earlier than in Ford Wood Beck. Such differences might be due to temperature. Unfortunately Harker was unable to obtain a permanent record and therefore no completely satisfactory comparison is possible, but she does indicate in another paper (Harker, 1953) that the temperature in the middle of the stream never rose above 140 C., in which case her stream was colder than Ford Wood Beck. 132 [March The dots in 1952 represent catches in an emergence trap, each dot being one adult. 1957] 133 There is a feature of fig. 2 that calls for further examination. In both the complete seasons, the histogram has a step-like character; in other words the population is growing denser as the season progresses. This is the opposite of what is to be expected, since it is known that Perla carlukiana eats Rhithrogena and there must be deaths from other causes too. Suspicion falls first on a faulty collecting technique that misses the smaller nymphs, with the result that more nymphs are caught as the number of large nymphs increases. AUG SEP OCT NOV DEC JAN 1953 JO-IImm . 8 -9mm. . 6- 7mm. . 4 -5mm. . 2 -3mm. .0-1 mm FEB MAR API MAY Fig. 3. Rhithrogena semicolorata. Upper histogram: Number of nymphs caught at station 2 with the shovel sampler. All are an average of 2 except the October catch which is an average of 4 and the August catch when only one sample was taken. Lower histogram: Nymphs from the same collections arranged in size-groups. The figures have been converted to percentages. Fortunately two collecting methods were used and the results obtained with the shovel sampler (fig. 3, upper histogram) are available for comparison with those obtained with the net. It is a pity that the two kinds of collection did not overlap completely in time, but the overlap was sufficient to show an entirely different pattern in the two. The shovel caught most nymphs at the beginning of the season, from which it may be deduced that the population is at a maximum then, as is to be expected, and that the net missed many small specimens. There remains the question: at what stage was the error made? Specimens caught by the shovel sampler were dislodged from the 134 [March stones by a flotation method, but the net-collector relied on the current to wash nymphs into the net. If nymphs were seen still clinging to a stone after it had been picked up off the bottom, it was swilled to and fro in the mouth of the net till they were no longer there, but it is possible that the smaller nymphs were not seen. Some could also have been overlooked during the subsequent sorting. But it seems much more likely that more tiny nymphs were caught by the shovel sampler than by the net because they were mainly on the small stones below the surface layer and not on the larger superficial stones which were the ones chiefly picked up by the net-collector. This is not, however, the complete story, as a glance at the lower part of fig. 3, in which the specimens are arranged in millimetre size-groups in successive months, will show. Nymphs only 1-2 mm. long occur in every month up to and including January. It is possible that some nymphs do not start growing till several months after hatching as do those of the plecopteron Leuctra fusca (Brinck, 1949). In the absence of observations on the eggs, comment must be cautious, but the writer inclines to the alternative that some eggs take much longer to hatch than others. Many must take four months, for the majority are laid in June and nymphs were never abundant before October, and it would seem that some take as long as seven months. This phenomenon of delayed hatching, or delayed growth in the smallest stages, it does not matter which for the present, seems, as will appear in later pages, to be rather common. It has considerable importance from two points of view. First it renders invalid the method of calculating growth rate from the average of monthly measurements used by Marker (1952) and lilies (1952 a and b), because this technique, as Brinck (1949) stresses, is applicable only to populations whose every member leaves the egg or starts to grow within a comparatively short time. Second, it complicates calculations of production. If we assume that each nymph requires what Andrewartha and Birch (1954) call “a place to live,” that is an area which provides food, shelter and perhaps other favourable conditions, and that every possible “place to live” is occupied, two extremes are possible. At one, each individual, whatever its size, has a secure retreat and mortality is low; at the other, the number of the “places to live” is inadequate and many of the nymphs that have failed to secure the best ones will fall a prey to predators and other hazards. In the first case, newly hatched nymphs will be forced to lodge in unsuitable places and soon perish; in the second there is a steady drain on all sizes of the population which is continuously made good from the influx of newly hatched specimens. A human sampler might find a population of similar size-structure on two occasions separated by an interval of time but would have no clue that, if the first set of conditions was effective, production was low, if the second, high. Possibly this phenomenon accounts for Allen’s (1951) observation that trout apparently ate about seventeen times more than there was to eat in the Horokiwi River. In other words, what Allen missed, and he must obviously have missed something, was the unhatched eggs of the invertebrates he was sampling. To revert to the growth of Rhithrogena semicolor ata , it is not possible to do more than gain a rough idea of the rate by looking at the size of the largest specimen in each successive catch. In general, in fig. 3, it has reached the next millimetre size-group each month. In the standard net catches in the 1957] 135 1 95 1 *2 season, the largest nymph was in the 3-4 mm. size-group in Septem¬ ber, in the 4-5 mm. group in October, and in the 6-7 mm. group in November, after which it was in the next group up in each month till March, when it had reached the 10-11 mm. group. After that there was no increase in maximum length. Results in the previous season were similar though less regular. It is not possible to make out whether the growth-rate was related to temperature, but it is incontrovertible that some nymphs at least are growing all through the winter. This affords a marked contrast to the findings of lilies (1952a, p. 481), according to whom Rhithrogena semicolorata did not grow in the months of November, December, January and February. When an attempt is made to compare the course of events at the different stations, a lack of information about certain points becomes evident. It would have been desirable, had it been possible in the time, to have had an emergence trap at each station, to discover the amount of egg-laying that was done in different parts of the beck, and to collect the smallest nymphs. These, which, as will be seen later, provided a clue essential for the unravelling of the life-history of Baetis rhodani , were taken in large numbers only by the shovel sampler, and this instrument was unfortunately not used at all the stations. However, it is possible to draw a few deductions reasonably well supported by facts. In 1950 nymphs were first collected in October, by which time most were 3-4 mm. long, the biggest catch being made at station 3 with the population at station 2 nearly as big and at the remaining stations distinctly smaller (table 1). In 1951 and 1952 at the same time the distribution of the population was similar though numbers at station 3 did not exceed those at other stations by quite so much. It seems likely, though confirmation by direct observation is to be desired, that the neighbourhood of station 3 is a favourite oviposition place. It is the most torrential part of the beck and Rhithrogena is a species typical of torrential conditions. Oviposition, however, is certainly not confined to this stretch, for it has been observed every year in Outgate Beck. Later on the percentage of the total catch at stations 2 and 3 tends to drop and this may be due to a dispersal of the specimens to stations both higher up and lower down. An analysis of the numbers of nymphs of different sizes at the various stations showed a higher proportion of larger nymphs in the populations of Outgate and Sykeside Becks than elsewhere. The chi square test was applied to a frame in which the number of specimens was divided horizontally into over and under 8 mm. long, and vertically into stations 1, 2 and 3 together and Outgate and Sykeside together. The result wras highly significant in both seasons. Further the average size of nymphs is consistently higher in both the tributary becks. This could be due to faster growth in them, but immigration from a more thickly populated area seems a likelier explanation. Presumably the larger nymphs travel further than the smaller ones. It had been hoped that the drying up of Sykeside Beck in 1951 would provide incontrovertible evidence about migration, but no nymph was found till November and it could have come from an egg laid by one of the last of the adults in early July when the beck filled up. The total population tends to increase steadily through the winter, but in April, 1951, and May, 1952, there was a big and inexplicable rise. In April B 136 [March of both years the increase in Sykeside Beck was about fivefold. In this beck there was above the station a long stretch that was never investigated, and it is possible that a migration downwards from here caused these large rises. There is evidence that there was some migration towards the mouth at this time of year for in May of both 1951 and 1952 the biggest population was at station 1, which at other times was generally less thickly inhabited than stations 2 and 3. A final point concerns the distribution of the largest specimens. Only 15 falling into the 11-12 mm. size-group were taken in the whole three years; of these 11 were in Sykeside or Outgate Beck. Outgate certainly and Sykeside Beck probably never get quite as warm as the main stream and possibly larger size is associated with temperature, for in the stream studied by Harker (1952), which may have had a lower maximum temperature, nymphs grew bigger than in Ford Wood Beck. Table i Numbers of Rhithrogena semicolorata caught with the coarse net in ten minutes St. 1 St. 2 St. 3 Ss Og Total 1950 March . . 39 78 100 13 24 254 May — 93 72 21 11 197 June 2 4 27 0 0 33 July . . 1 0 0 0 0 1 August 0 0 0 0 0 0 September 0 0 0 0 0 0 October 13 50 73 18 11 165 November 16 62 55 3 6 142 1951 January 59 141 116 29 13 358 February 77 143 120 34 31 405 March 125 122 128 16 39 430 April 123 227 220 156 43 769 May 245 159 146 66 59 675 June 23 0 0 0 0 23 July . . 1 0 0 0 0 1 August 0 0 0 0 0 0 September 0 1 6 0 0 7 October 3 10 15 0 0 28 November 16 37 37 1 8 99 December 34 78 64 1 19 196 1952 January 5i 99 100 3 35 288 February 79 hi 48 6 46 290 March 50 9i 181 13 58 393 April . . 94 151 4i 57 47 390 May* 178 172 156 10 42 558 June 10 15 4 4 9 42 July* 0 2 0 0 0 2 August 0 4 0 0 3 7 September 3 9 25 3 11 5i October 55 64 102 29 39 289 November 182 227 396 48 118 971 December 164 164 107 42 102 579 * Numbers caught with the fine net in five minutes and doubled. 1957] 137 Ecdyonurus torrentis Kimmins Rawlinson (1939) showed that E. venosus , in the R. Alyn, had a quick summer generation and a longer one that overwintered. Harker (1952) found that E. torrentis also had a quick summer generation, but the life-history was a little more complicated than that elucidated by Rawlinson, because there were three not four generations in two years. The progeny of the summer generation emerged in May of the following year, their progeny in March the year after that, and the third generation completed development within the summer again, emerging in July. Table 2 Numbers of Ecdyonurus caught with the coarse net in ten minutes St. 1 St. 2 St. 3 Ss Og Total 1950 March . . 59 65 28 2 2 156 May — 40 11 2 2 55 June 2 15 13 0 — 30 July 14 6 0 0 0 20 August . . 0 I 1 0 0 2 September 16 26 8 1 2 53 October 93 102 19 20 8 242 November 66 20 40 8 5 139 1951 January in 7 14 29. 11 172 February 43 22 8 11 7 9i March 103 23 45 12 13 196 April 21 13 4 4 3 45 May 53 15 17 7 5 97 June 66 68 5 — 0 139 July . . 26 5 0 0 0 3i August 7 15 11 0 0 33 September 4 56 10 5 2 77 October 36 29 7 13 5 90 November 77 103 8 1 17 206 December 65 9i 7 5 11 179 1952 January 44 36 4 7 15 106 February 58 6 4 3 10 81 March 3i 5 9 8 12 65 April 26 22 2 1 8 59 May* 58 8 2 0 , 0 68 June 13 29 1 0 8 51 July* 10 0 0 0 0 10 August 6 16 7 6 10 45 September 13 109 70 29 72 293 October 16 63 55 14 3i 179 November 44 125 135 30 61 395 December 27 39 27 14 22 129 * Numbers caught with fine net in five minutes and doubled. In Ford Wood Beck E. torrentis has a life-history like neither of these, but a simple single-generation-a-year one. Fig. 4 shows the actual numbers taken with the phytoplankton net; collections made in the other years confirm that this is typical. It includes the small ones that could not be identified to species, since the only other possibility, E. venosus , was very scarce, and the likelihood that it produced many young, nearly all of which died when very small, is remote. During the winter, there is a big size range, apparently because the population is continuously augmented by small nymphs, as in Rhithrogena. Small nymphs continue right up to May, but none could have 138 [March come from eggs laid the same year, as Harker found, because adults were never seen before May. Emergence continues till September. Tiny nymphs appear in August and grow fast, but not fast enough to emerge the same year. If any specimens complete a generation within the summer, the number must be very small. Again, it is not possible to calculate growth in the traditional way, because the population is continually being added to. The shape of the histograms and the size of the largest specimen each month in 1950 and 1951 indicate that growth is rapid in the autumn, but slow during the winter. The life-history of Ecdyonurus torrentis is similar to that of Rhithrogena semicolorata except that emergence straggles over the summer and is not confined to a period at the beginning of it. The proportion of nymphs of different sizes was almost exactly the same in shovel and phytoplankton net samples, whereas proportionately many more nymphs of Rhithrogena under 2 mm. were taken by the shovel than by the net, a difference that must reflect a difference in habitat. The tiny nymphs of Rhithrogena were thought to cling to the smaller stones, and therefore the presumption from these observations is that those of Ecdyonurus do not. Numbers were always relatively low in August. This was to be expected in the month when the last of the old generation had gone and the new had only just appeared. In the next four months, the population was generally highest at station 2; to be exact, in eleven catches the population was highest here eight times, at station 1 twice and at station 3 once. In the other months of the year, by contrast, the population was nearly always greatest at station 1, often by a considerable amount. It was highest elsewhere, in fact station 2, only four times in seventeen catches and, inexplicably, three of the four were in June (table 2). It is not possible to offer an explanation of these observations. That the new generation was most abundant at a point further upstream than the old generation, could have been due to movements of the adults before the eggs were laid or to movements of the nymphs after the eggs had hatched. The point cannot be settled without observations on the eggs, but no such observations were made. The concentration at station 1 after the turn of the year could have been due to migration downstream or to greater mortality higher up that at station 1. The actual numbers taken do not illuminate this problem, because big fluctuations from catch to catch are typical of Ecdyonurus torrentis. Dr. Janet Harker, in a painstaking piece of work involving marking nymphs and making frequent collections, found an upstream migration of nymphs of Ecdyonurus torrentis during November. These observations have not been published, but were described at a meeting of the Ecological Society of which there is an account on p. 418 of vol. 22 of th z Journal of Animal Ecology. There was probably upstream migration in Ford Wood Beck, because the proportion of nymphs over 9 mm. long was higher in the stations furthest upstream, as shown by the following figures : Number of specimens collected Season 1950-1 Season 1951-2 Size St. 1 St. 2 St. 3 Ss Og St. 1 St. 2 St. 3 Ss Og Over 9 mm. 86 80 17 32 13 63 3i 7 13 39 Under 9 mm. 504 222 143 36 40 349 344 62 3i 45 13-14 1957] 139 Actual numbers of Ecdyonurus torrentis caught with the fine net arranged in mm. size-groups (season 1951-2). 140 [March The differences are highly significant. Heptagenia lateralis (Curtis) September was the month in which small nymphs were first found in fair numbers in both 1951 and 1952 (fig. 5). There was some growth in the autumn, little in the winter, and a start again in February or March. Fair numbers were caught each month until June, 1952, and July in 1951, when a big drop was presumably caused by emergence. This species was never taken in the cage at station 1, which was not unexpected because most of the nymphs were in the upper parts of the stream. To judge from the nymphs, the emergence period was short but, because numbers were not large, it is impossible to be certain about this. In the beck studied by Harker (1950) the species differed in exactly the way Rhithrogena semicolorata did, that is the nymphs grew larger, the adults appeared later, and the eggs hatched sooner. Heptagenia lateralis resembles Rhithrogena semicolorata and Ecdyonurus torrentis in having one generation a year but it grows less in the winter than Rhithrogena and does not start to emerge until later than both of them. In 71 out of a total of 93 catches, fewer than ten specimens were collected. Only 6 times did numbers exceed twenty and the highest was 76. In this collection, made in Outgate Beck in September, numbers were high in both halves of the coarse net collection and in the phytoplankton net collection, which suggested that nymphs were abundant over a fairly large area. They were not particularly abundant in the following month. On the other occasions, the large numbers were in only one of the collections, which suggested that the nymphs aggregated in peculiar local conditions that were only rarely disturbed by the collector. Heptagenia lateralis occurred at all the stations but the greatest numbers were at station 3, Sykeside and Outgate. Baetis rhodani (Pictet) Harker (1952) was unable to work out the life-history of B. rhodani from her figures, which excluded specimens less than 2 mm. long. The present series of collections was no more fruitful till, in the autumn of 1951, a fine net was brought into use in addition to the coarse one. As a result, enormous numbers of tiny nymphs, which passed through the coarse net, were captured, and it is these tiny nymphs that provide the clue to the life-history of B. rhodani. The error of the coarse net is discussed in another publication and here it suffices to recapitulate that the two nets caught roughly equal numbers of specimens 5-6 mm. long and the coarse net caught about half the specimens 4-5 mm. long, one in six of those 3-4 mm. long, one in thirty of those 2-3 mm. long, and hardly any of those under 2 mm. long. To elucidate the life-history we have therefore: 1 . The catches in the fine net, some of which are so large that it is prefer¬ able to present the actual figures rather than histograms, which is done in table 3. 1957] I4i I o 3t N I On d o CO Cd Jd 4-1 ■3 •fH £ .d 50 d cd o CO *-< 4> J=> d d cd d 4-> o < »n bh • H Table 3 mthly catches with the fine net of Baetis rhodani arranged in size groups 142 [March d VN r- N VN ro 0 0 O O 0 o> (NJ t'' On NO Q (S VN ro M > ro ro 00 VN VO On VN On ro O O 0 ro 00 ON 00 00 t-' M M IH n M H 00 ro M ON VN tH O t" On r^ VH VN 0 00 NO t}- ro NO (S O M O O >N M V VO NO VN VN 3 )H M VN VN M -9 - 2-J Ak mAy jUn jul ' 1 Fig. 8. Actual numbers of Paraleptophlebia submarginata caught with the coarse net arranged in mm. size-groups (season 1951-2). Both Harris (1952, p. 64) and Sawyer (1950) record that this species flies upstream before egg laying. Oviposition in Ford Wood Beck has not been observed but any upstream migration seems unlikely because most of the nymphs occur at station 1. Upstream flight over a stony stream is perhaps not to be expected since Sawyer’s observations are that the females lay eggs in broken water and suggest that the flight upstream may continue only till broken water is found. Fig. 9. Total number of Ephemerella ignita caught with the coarse net in 1951. 1957] 153 Discussion Of these seven, only one, Ephemerella ignita, is a true summer species, growing during the warmest time of the year and lying dormant for the rest. All the others grow in autumn and spring, if not throughout the winter, and Rhithrogena semicolor ata, Heptagenia lateralis and Paraleptophlebia sub- marginata have a period of inactivity during the middle of the summer ; the stream therefore is inhabited mainly by cold-water species. In the stream studied by Harker, the period of inactivity was shorter, probably because the stream did not get so warm, and was curtailed at both ends, adults emerging later and eggs hatching earlier. It is possible that an advancement of the date of hatching could lead to a situation where conditions permitted the com¬ pletion of a quick summer generation. This variation in the life-history in two different streams raises several questions and it is worth noting comparable observations; no experimental work has been done and it is not possible at present to do more than speculate about the causes, but it may serve a useful purpose to examine the hypotheses of other workers. Brinck (1949, pp. 142-4) shows that the emergence of many species of Plecoptera is progressively later in colder zones in Sweden. Nearly all the species emerge in the first half of the year, lay eggs and die. The eggs hatch after a fairly short time but the nymphulae that emerge from them do not grow during the high temperatures of summer and show no increase in size till the autumn. Growth is slowed down by the low temperature of mid¬ winter and therefore specimens take longer to develop in the north than in the south and emerge later. The significant part of this life-history is the nymphula, which can tolerate quite high temperatures but which does not grow until the waters become cold in autumn. Ide (1935) observed that Iron pleuralis in a Canadian river emerged from mid-April to mid-August near the spring, but only from mid-May to mid- June lower down. His explanation of the restricted emergence period in the lower warmer parts of the river is a very simple one. He postulates that, for a period during the summer, temperature is high enough to kill all but eggs. These take a long time to develop and any that hatch too soon give rise to nymphs doomed to succumb to a lethal temperature. This will mean that the first adult will appear later in the following year than in parts of the river that are colder in summer. The emergence period cannot go on as long because any nymph that has not emerged by a certain time will be killed by high temperature. Ide also points out that, in a stream with little annual tempera¬ ture fluctuation, the breeding period will be restricted by a winter air temperature too low for the adults to mate. Pleskot (1953) describes how Habroleptoides modesta , a common European species that does not occur in Britain, has a flight period restricted to early summer, though small nymphs are present all through the summer. She argues that summer temperatures cannot therefore be lethal but suggests that at them there is not enough oxygen for nymphs that, about to metamorphose, are in a critical stage when transference of oxygen from medium to tissues is difficult. It is oxygen lack at high temperatures that restricts emergence to early summer. 154 [March There are thus two possible explanations of the curtailment of the emergence period at its latter end — a direct temperature effect according to Ide and an indirect temperature efFect according to Pleskot. Both may be right; only experiment can show. There are also two possible explanations of what prevents nymphs appearing earlier in summer than they do. Again Ide postulates a direct temperature effect* but the hatching of the egg may be determined by something within it and it could be a stage, like the plecopteran nymphula, that will not develop further until conditions are favourable. Ide mentions heredity but says little about it; it seems likely that mortality among nymphs that have come out of the egg too soon or have failed to complete development in time becomes reduced quite soon because selection produces a strain with a life-history adapted to the local conditions. It is clear, for example, that Rhithrogena avoides emergence of adults too early by what is probably a diapause, for some nymphs are full grown in mid-winter but none emerge before May. Whatever causes the kind of life-history under discus¬ sion and its variation from place to place, there is little doubt that it is ultimately due to temperatures that reach an unfavourably high level in summer. Summary 1. Collections were made every month for three years in a small stony stream in the Lake District. 2. Rhithrogena semicolorata emerges mainly in late May and June, the new generation begins to appear in August, September or October and growth proceeds throughout the winter, development being completed in one season. Tiny nymphs are found up to January, which is thought to be due to delayed hatching of eggs, a phenomenon of considerable importance in the calculation of growth rate and productivity. In a stream studied by Harker, which was probably colder, emergence went on longer, eggs hatched sooner, and nymphs grew bigger. Most eggs are perhaps laid in the swiftest reach of the stream, from where the nymphs move upstream and down. They tend to move downstream before emergence. 3. Ecdyonurus torrent is has a life-history similar except that emergence takes place throughout the summer. This species is most plentiful at the lower end of the stream. 4. Heptagenia lateralis is also univoltine but it grows less in winter and emerges later than Rhithrogena semicolorata. It is most abundant in the upper parts of the stream. In the stream studied by Harker it differed in exactly the same way as Rhithrogena semicolorata. 5. Baetis rhodani has a long overwintering and a quick summer generation, the former emerging from April to July, the latter from July to November. It seems likely that each can be divided into two according to whether the members or their parents emerged from the egg in autumn or in mid-winter, though it is not possible to be certain of this until more is known about delayed hatching of eggs and what causes them to hatch ultimately. The 1957] 155 adults of the summer generation are smaller than the others. There was a concentration of nymphs at the lower end of the beck some time during each of the three summers. 6. Baetis pumilus has a similar life-history, but the emergence period is shorter. 7. Paraleptophlebia submar ginata adults appear in May and June, tiny nymphs in August, and growth slows down in the middle of the winter. 8. Ephemerella ignita nymphs are found only in June, July, August and occasionally September, and the species is thought to spend the rest of the year dormant in the egg stage. 9. Rhithrogena semicolorata , Heptagenia lateralis and Paraleptophlebia sub- marginata have a resting period in the summer, probably spent in the egg stage. It seems likely that it tides over a period of unfavourably high temperature. The theories of other workers are discussed. Temperature may have a direct lethal effect, or oxygen may be deficient in the warm waters of summer. However temperature exerts its influence originally, it seems likely that natural selection soon produces a strain adapted to local conditions. References Allen, K. R., 1951. The Horokiwi stream. A study of a trout population. Fish . Bull. Wellington , N.Z. , No. 10: 1-23 1. Andrewartha, H. G., and Birch, L. C., 1954. The distribution and abundance of animals. London: Cambridge U.P. Brinck, P., 1949. Studies on Swedish stoneflies. Opusc. ent. suppl., 11: 1-250. Butcher, R. W., Longwell, J., and Pentelow, F. T. K., 1937. Survey of the River Tees, Pt. 3. The non-tidal reaches — chemical and biological. Tech. Pap. Wat. Pollut. Res. , Lond., No. 6: 1-189. Clegg, J., 1952. The freshwater life of the British Isles (Wayside and Woodland series). London: Warne. Gillies, M. T., 1950. Egg laying of olives. Salm. Trout Mag., No. 129: 106-8. Harker, J. E., 1952. A study of the life cycles and growth-rates of four species of mayflies. Proc. R. ent. Soc. Lond. (A), 27: 77-85. - 1953* An investigation of the distribution of the mayfly fauna of a Lanca¬ shire stream. J. Anim. Ecol., 22: 1-13. Harris, J. R., 1952. An Angler's Entomology (New Naturalist series). London: Collins. Ide, F. P.j 1935. The effect of temperature on the distribution of the mayfly fauna of a stream. Publ. Ont. Fish Res. Lab., No. 50: 1-76. Illies, J., 1952a. Die Molle. Faunistisch-okologische Untersuchungen an einem Forellenbach im Lipper Bergland. Arch. Hydrobiol., 46: 424-612. - 1952b. Die Plecopteren und das Monardsche Prinzip. Ber. Limnol. Flussst. Freudental., 3: 53-69. 156 [March Macan, T. T.j 1950. Descriptions of some nymphs of the British species of the genus Baetis (Ephem.). Trans . Soc. Brit. Ent., 10: 143-66. Moon, H. P., 1939. The growth of Coenis horaria (L.), Leptophlebia vespertina (L.), and L. marginata (L.) (Ephemeroptera). Proc. zool. Soc. Lond. (A), 108: 507-12. Percival, E., and Whitehead, H., 1928. Observations on the ova and oviposition of certain Ephemeroptera and Plecoptera. Proc. Leeds, phil. lit. Soc., 1: 271-88. - 1930. Biological survey of the River Wharfe, 2. Report on the invertebrate fauna. J. Ecol ., 18: 286-302. Pleskot, G., 1953. Zur Okologie der Leptophlebiiden (Ins. Ephemeroptera). Ost. zool. Z., 4: 45-107. Rawlinson, R., 1939. Studies on the life-history and breeding of Ecdyonurus venosus (Ephemeroptera). Proc. zool. Soc. Lond. (B), 109: 377-450. Sawyer, F. E., 1950. B.W.O. Studies of the Sherry Spinner. Salmon and Trout Mag., No. 129: 1 10-4. - 1953- The blue-winged olive. Field, 201:1038. {Continued from inside front cover) ORTHOPTERA, Etc. A Summary of the Recorded Distri¬ bution of British Orthopteroids. By D. K. McE. Kevan, 1952. 16 pp., 5 s. od. HEMIPTERA-HETEROPTERA The Natural Classification of British Corixidae. By G. A. Walton, 1943. 14 pp., 15. 9 d. Contributions towards an Ecological Survey of the Aquatic and Semi-Aquatic Hemiptera- heteroptera of the British Isles. Anglesey, Caernarvon and Merioneth. By E. S. Brown, 1943. 62 pp., 35. od. North Somerset. By G. A. Walton, 1943. 60 pp., 26 figs., 45. od. Scottish Highlands and East and South England. ByE. S. Brown, 1948. 45 pp., 75. 6 d. The Ribble Valley (Lancashire South and Mid). By E. J. Popham, 1949. 44 pp., 1 map., 85. od. North-East Wales (Denbighshire and Merionethshire). By E. J. Popham, 1951. 12 pp., 25. 6d. The Hemiptera-Heteroptera of Kent. By A. M. Massee, 1954. 36 pp., 75. 6d. The Bionomics and Immature Stages of the Thistle Lace Bugs {Tingis ampliata H.S. and T. cardui L.; Hem., Tingidae). By T. R. E. South- wood and G. G. E. Scudder. 85. od. COLEOPTERA The Aquatic Coleoptera of North Wales. By E. S. Brown, 1948. 15 pp., 1 fig., 15. od. The Aquatic Coleoptera of Wood Walton Fen, with some compari¬ sons with Wicken Fen and some other East Anglian Fens. By F. Balfour-Browne, 1951. 36 pp., 45. 6d. LEPIDOPTERA The Morphology of Luffia ferchaul- tella and a Comparison with L. lapidella (Psychidae). By R. S. McDonogh, 1941. 19 pp., 9 pis., 45. od. List of the Lepidoptera of Dorset. Part 2. By W. Parkinson Curtis, 1947. 138 pp., 4 pis., 1 15. od. Postural Habits and Colour-Pattern Evolution in Lepidoptera. By M. W. R. de V. Graham, 1950. 16 pp., 4 pis., 4 figs., 45. od. HYMENOPTERA A Consideration of Cephalic Struc¬ tures and Spiracles of the Final Instar Larvae of the Ichneumoni- dae. By B. P. Beirne, 1941. 68 pp., 31 figs., 55. 6 d. Second Review of Literature con¬ cerning British Ichneumonidae. By G. J. Kerrich, 1942. 35 pp., 7 figs., 35. od. The Hymenoptera Aculeata of Bed¬ fordshire. By V. H. Chambers, 1949. 56 pp., 3 maps, 105. od. An Introduction to the Natural History of British Sawflies. By R. B. Benson, 1950. 98 pp., 9 pis., 105. od. Notes on Some British Myrmaridae. By W. D. Hincks, 1950. 42 pp., 5 figs., 1 pi., 55. od. The British Species of the Genus Ooctonus Haliday, with a Note on some Recent Work on the Fairy Flies (Hym., Myrmaridae). By W. D. Hincks, 1952. 12 pp., 8 figs., 45. od. The Natural History of some Pamphilius Species (Hym., Pam- philiidae). By V. PI. Chambers, 1952. 16 pp., 4 pis., 55. od. A Study of some British species of Synergus. By J. Ross, 1951. 16 pp., 45. od. A Revision of Section I (Mayr, 1872) of the Genus Synergus (Hym., Cynipidae) in Britain, with a Species new to Science. By R. D. Eady, 1952. 12 pp., 4 pis., 45. od. The British Ants allied to Formica fusca L. (Hym., Formicidae). By I. H. H. Yarrow, 1954. 16 pp., 8 figs., 3 maps, 55. od. The British Ants allied to Formica rufa L. (Hym., Formicidae). By I. H. H. Yarrow, 1955. 48 pp., 58 figs., 1 map, 105. 6d. DIPTERA Preliminary List of the Hosts of some British Tachinidae. By H. Audcent, 1942. 42 pp., 25. 9 d. An Outline of a Revised Classifica¬ tion OF THE SYRPHIDAE (DlPTERA) ON Phylogenetic Lines. By E. R. Goffe, 1952. 28 pp., 3 figs., 65. od. A Revision of the British (and notes on other) Species of Lonchaeidae (Diptera). By J. E. Collin, 1953. 28 pp., 3 pis., 3 figs., 65. od. Orders, accompanied by the appropriate remittance, should be addressed to the Hon. Secretary CONTENTS T. T. Macan : The Life Histories and migrations of the Ephemeroptera in a stony stream. Communications for the Transactions should be sent to: E. J. Popham, Department of Zoology, The University, Manchester, 13 The author of any published paper shall, if he so request at the time of communicating such paper , be entitled to receive twenty-five copies thereof gratis Information regarding the Society may be obtained from the Secretary, J. G. Blower, Department of Zoology, The University, Manchester, 13 Printed by Sydenham & Co. (Est. 1840) Ltd., Printers, Oxford Road, Bournemouth MCZ ERNST MAYR LIBRARY 3 2044 128 447 547 Date Due