Macan, T.T.

English

Aquatic Commons

47 
INVERTEBRATE ANIMALS OF THE 
LAKE DISTRICT. 
by T. T. Macan. 
Substantial contributions towards the preparation of a list of the 
animals occurring in Windermere and its neighbourhood have now 
been made and a stage has been reached where it is necessary to fix 
some boundary within which the work may be confined. The county, 
the unit most commonly used, is not satisfactory for Wray Castle is 
in Lancashire and Windermere is in Westmorland, and the boundary of 
a third county, Cumberland, is not far away. Apart from this a county 
is seldom a good unit for it contains different kinds of country and, 
therefore, many different kinds of habitats for the fauna. It is better 
to work over an area where conditions are fairly uniform, and where 
there is a clear natural boundary. The area known as the "Lake 
District" fulfils both these conditions, and has been selected as the unit 
for ecological work centred at Wray. There are three main types of 
rock, Skiddaw Slates, Borrowdale Volcanics, and Bannisdale Slates, 
but all agree in being hard and deficient in lime. The number of 
features in which habitats, particularly those of freshwaters, can vary 
are therefore limited, and for this reason it is all the more profitable to 
study the extent to which such variations as do exist affect the fauna. 
The boundary of the Lake District is a ring of Carboniferous 
Limestone which surrounds it on all sides except the southwest where 
there are beds of Permian and Trias. There is also a small gap to the 
east over Shap Pass where the Lake District rocks are continuous with 
those of the Pennines and here it is necessary to make an arbitrary 
boundary; I have used the main road from Kendal to Shap. This 
definition is taken from Marr (1916) and figure 7 is based on the map 
in his book, "The Geology of the Lake District." 
The distribution of Corixidae in the Lake District, described in 
the last report, has now been written up and accepted for publication 
by the Journal of Animal Ecology. This study has also been extended 
to southern England during a fortnight's collecting leave at South 
Haven Peninsula, Studland, Dorset, where Captain C. Diver's 
ecological survey has made unnecessary the usual preliminary labours of 
finding localities and identifying plants, geological formations, etc. 
It was possible with two colleagues to collect a considerable amount of 
data during this short period, and the results, which provide a very 
48 
satisfactory corroboration of conclusions drawn from work in the Lake 
District, are the subject of a second paper. 
Several other groups were collected in the Lake District at the 
same time as the Corixidae, and these have now been submitted to 
different authorities, who have very kindly named them, and I would 
like to take this opportunity of thanking Professor A. E. Boycott and 
Mr. G. I. Crawford (snails), Mr. C. Oldham (Pisidium), Professor F. 
Balfour-Browne and the Rev. E. J. Pearce (water-beetles), and Mr. W. 
Williamson (water-mites). 
FIGURE 7. The Lake District as defined for faunistic studies. The boundary 
is marked by a heavy line, county boundaries by broken lines and the position 
of Wray Castle is indicated by
49 
The snail fauna of the district is poor and I have only found 15 
species, though a few more are recorded in the literature. Only six 
of these are really common but preliminary investigations suggest that 
the elucidation of the factors which determine their distribution 
should be of interest not only to ecologists but also to fishermen, for 
snails are an important article in fish diet. Of the six common species 
Planorbis carinatus is confined to Windermere; Planorbis contort us, 
Physa fontinalis and Valvata piscinalis are found only in large lakes and 
in certain large low-lying bodies of water which, though commonly 
called tarns, are biologically lakes also; Planorbis albus is found in 
lakes and also in a number of tarns where the carbonate hardness of the 
water is relatively high, and Limnaea pereger is found in all localities 
except those where the carbonate hardness is extremely low. Dr. 
C. H. Mortimer has kindly estimated the carbonate hardness of a large 
number of water samples for me. It would appear that this factor, 
which in the district is probably largely due to calcium, may be limiting 
for Planorbis albus and Limnaea pereger, but since the value of the 
carbonate hardness of some of the tarns is greater than that of the lakes, 
some other factor must limit the distribution of the remaining four 
species of snail. Planorbis carinatus is of particular interest because 
in other parts of England it is usually found only in waters with at 
least 20 mg. of calcium per litre; Windermere has 6 to 8 mg. It 
appears therefore that for this species, as well as for the other three 
species common in lakes, a factor of importance in distribution 
is related to the actual size of the water-body. In this district the 
factor is not likely to be competition, for everywhere the snail 
populations are sparse. 
Another species usually associated with a high calcium content, 
Limnaea stagnalis, has been found in one small tarn where the car-
bonate hardness is higher than is usual in the district. A further 
interesting discovery was Hydrobia jenkinsi, a species which is known 
to be invading fresh water from brackish. It has been found in one 
bay in Windermere, and the distribution has been carefully mapped 
with the assistance of Miss E. Carey so that comparison may be made 
in a few years' time. The distribution to-day suggests a dying popu-
lation, but if the population proves to be on the increase the species is 
likely to have a profound effect on the biology of the lake, for in one 
place over 1,500 specimens were found in one square foot. Hydrobia 
jenkinsi is rather smaller than Limnaea pereger, but not too small to 
50 
provide suitable food for trout and other fish, so if it should spread 
throughout Windermere and to other fresh waters, the food supply 
for fish might be considerably modified and perhaps improved. 
Eleven species of Pisidium (sometimes known as pea-shells), 
have been collected, and eight of these are common. P. milium, P. 
hibernicum, and P. lilljeborgii have been found abundantly in Winder-
mere, and P. nitidum and P. subtruncatum have been found abundantly 
both in Windermere and in various tarns. All, however, are so closely 
associated one with another that it has not been possible to detect any 
individual ecological requirements. P. cinereum, P. personatum and 
P. obtusale are found more abundantly in tarns and pools than in 
Windermere, P. cinereum often at high altitudes. It is noteworthy 
that Miss C. Humphries in her work on the profundal fauna of 
Windermere records that P. cinereum and P. personatum are dominant 
at great depths. An interesting species is P. conventus, which has 
been found in Red Tarn (Hellvellyn) and in another small tarn in the 
next valley, thereby confirming a doubtful record of Mr. Oldham's. 
It was previously known certainly only from high localities in Ireland 
and Wales. 
Fifty-one species of water beetle, belonging to the families 
Dytiscidae and Haliplidae, have been found. Professor F. Balfour-
Browne has very kindly supplied a list of references, and he and Mr. 
F. H. Day and the Rev. E. J. Pearce have put at my disposal a number 
of hitherto unpublished records. A paper incorporating all these data 
is nearly complete. The Dytiscidae are all carnivores, and the larger 
species may take a heavy toll of fish fry. In the Lake District, however, 
beetles are not abundant, and the larger ones have only been found in 
small pools. They occur where there is thick cover of water-weeds, 
particularly if it is associated with vertical banks extending to some 
depth, and these conditions are not found in the larger lakes and streams. 
A number of other groups of invertebrates have been studied by 
previous workers, notably Platyhelminthes by Messrs. Beauchamp and 
Ullyott, Polyzoa by Dr. A. Hastings, Rotifera by Mr. David Bryce, and 
Crustacea by Mr. D. J. Scourfield, but we are still not in a position to 
prepare a complete fauna list because the aquatic larvae of insects such 
as the ephemerids, caddis-flies and two-winged flies cannot yet be 
reliably identified. A number of these insects have now been bred in 
the laboratory to associate the larval forms with adult insects. Mr. D. 
E. Kimmins of the British Museum kindly identified a collection of 
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adult Ephemeridae obtained in this way. There proved to be six 
species only, all belonging to different genera. It is felt that satis-
factory taxonomic descriptions can only be made when the larvae of 
many species in each genus are available for comparison, so workers in 
other parts of the country are invited to send collections of living 
ephemerid larvae to Wray Castle to be bred in the laboratory. 
Several species of caddis-flies and stone-flies have been reared 
and also 10 species of dragon-flies. Although keys for the identification 
of dragon-fly larvae exist, they depend on small numerical differences, 
and it seems desirable to examine many examples of each species to 
confirm the validity of such differences. This work on aquatic larvae 
will be continued during the coming year. 


Invertebrate animals of the Lake District

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Invertebrate animals of the Lake District

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