64 GNATS OR MOSQUITOES — CHAPTER IV 



carrying a number of strong radiating bristles, the ana- 

 logue of the whorl-organ of the other genera. The 

 mandibles are very large and trenchant, and being capable 

 of very wide abduction are particularly well adapted for 

 seizing the larva's prey. The maxillae are two small, very 

 simple plates, and the lower lip can hardly be said to be 

 represented as such. 



The thorax, which is much the stoutest part of the body, 

 is of fusiform outline, and shows little or no indications 

 of its component segments ; both this and the abdomen 

 are provided with only a few small compound bristles. 

 The abdomen is composed of nine segments, which pro- 

 gressively increase in length to the seventh. These are 

 of nearly equal width to the sixth, after which the body 

 rapidly tapers off. The anal tubercles and swimming 

 fans are small, but round the anus are four bristles of 

 much greater size. Eound the anus also arc several ranks 

 of peculiarly-shaped hooks. 



The respiratory system is peculiar. Apparently the 

 function must be entirely aquatic, as in the very young 

 larvce there are no signs whatever of tracheae, and even in 

 the fully-grown creature there are no stigmata or external 

 breathing apertures whatever. In young larvae all that 

 can be seen are the two pairs of air sacs, which are 

 situated in the same positions as in Mochlonyx, but pre- 

 sent a very different appearance as they contain no air, 

 but are full of serum and are lined with a large-celled 

 epithelium with prominent nuclei. Gradually with suc- 

 cessive changes of skin the main longitudinal trunks and 

 their branches appear piecemeal. At first they are full of 

 serum, but as development proceeds they gradually fill with 

 air, which, however, must be secreted from the blood, as 

 there is at no period of larval life any direct communication 

 with the exterior. 



The pupa in this genus is distinguishable by the rela- 

 tively large size of the abdomen and the small dimensions 

 of the cephalo-thoracic mass. As far as its respiratory 

 arrangements are concerned, however, it closely resembles 

 the pupae of the other genera, the breathing trumpets being 

 well developed, with very oblique mouths. 



