ON THE BRITISH SPECIES OF SIMULIUM — ^11. 213 



in sluggish and permanent rivers such as those of East Anglia ; (2) those which live 

 on rocks, stones (chiefly the upper sides) and sometimes on water-plants in swift 

 mountain streams, which may become dry in dry seasons ; and (3) those which live 

 chiefly on the undersides of stones, in small temporary streams, which are dry for 

 several months of the year. These classes are not rigidly defined, either in regard 

 to the water-courses or to the insects, but it is often possible to say from the 

 appearance of a stream what species will be found in it. Some, for example S. latipes, 

 are more particular in their choice of a locality than others, such as S. ornatum, 

 which is perhaps the most catholic in its tastes. The different species found in one 

 type of stream are nob all closely related, as is shown by a study of the adults and 

 pupae ; but it is a noteworthy fact that most of the larvae which live in very rapid 

 streams have their anal gills very much branched, while most of those which live in 

 sluggish rivers have the three main branches simple. It might have been supposed 

 that the more rapidly- moving and presumably more highly aerated water would not 

 need such a large surface in the gills for the insects to accomplish their respiration, 

 but this is not what we actually find. It may be that the presence of water-plants 

 assists respiration to an even greater extent than movement of the water. 



Larvae always occur in the greatest numbers in the swiftest part of whatever 

 stream they are in. The slow-river species furthermore show a decided preference 

 for those water-plants (e.g., Scirpus, Sparganium and Ranunculus) which have long 

 and narrow or much divided leaves, the reason perhaps being that such leaves are 

 waved about by the current and thus provide better aeration ; the larvae occur 

 most abundantly towards the tips of the leaves. 



The species found on stones all show a decided preference for clean stones ; 

 when there is much moss or algal growth or^ccumulation of sediment on the stones 

 Simulium larvae are not often found in numbers. This may perhaps be due to 

 the growth affording shelter for predaceous insect enemies. It is also very noticeable 

 that Simulium larvae are never very abundant ia those stony streams where there 

 are large numbers of a certain small caddis- worm which lives in pebbly cases attached 

 to the stones. 



Larval Moults and Groivth Changes. —I have been able to follow the entire larval 

 development in only one species, S. latipes, but it will probably be safe to regard this 

 as typical of all in respect to the number of moults and the principal developments 

 which take place in the larval stage. In this species there are six moults, the change 

 to the pupa taking place at the sixth. The first-stage larva is readily distinguishable 

 from the others by the presence of the egg-burster, a small chitinous tooth near the 

 posterior end of the clypeus, it being much more strongly developed* in S. latipes and 

 S. aureum than in S. venustum (?) ; in the former it is plainly visible under a f"" 

 objective as a dark spot in the middle of the back of the head ; in the latter it is only 

 discernible with a higher power. The first-stage larva is also characterised by one 

 or two pairs of small hairs on each side of the egg-burster , which disappear after the 

 first moult ; by the smaller number of elements in the mouth-brushes and in the 



_. * The actual tooth is no larger, but is darker in colour and surrounded by a darker 

 and more strongly chitiuised area of the general integument of the head. In this respect 

 Simulium differs strikingly from the Culicidae, in which the egg-burster is surrounded by 

 a membranous area. 



