132 (INATS OR MOSQUITOES — CHAPTER VII 



humped-up thorax forms the highest part, and it is from 

 the dorsal surface of that part of the body that the 

 respiratory syphons spring, being in the form of two horns 

 or asses' ears. Their upper extremity is cut obhquely, and 

 when the pupa is at rest is always kept above water. 

 The eyes are distinct, and beneath the thorax is found 

 a large mass consisting of the antennae, mouth parts and 

 legs. The abdomen is elongated, segmented, and ter- 

 minated by two oval plates. If the nymph desires to go 

 beneath the surface of the water it straightens itself and 

 gives a few strokes of the tail, but is soon carried back to 

 the surface, as soon as this has ceased to act, by its own 

 buoyancy. After passing five or ten days in this state, the 

 insect is ready for its last metamorphosis, but this is a most 

 critical period of its life, as if the nymph case upsets during 

 the process of the imago's freeing itself, the insect perishes 

 by drowning, as it is now entirely unable to survive con- 

 tact with the element in which, up to now,, it has passed 

 its life. The moult that frees the imago from the nymph 

 integuments takes place in the same manner as the pre- 

 ceding ones ; a rent appearing in the upper surface of the 

 thorax through w^hich the gnat protrudes first the head 

 and thorax as much as possible above the aperture. The 

 posterior extremity of the body now contracts a little, and 

 extending itself immediately after, is gradually drawn out 

 in a perpendicular plane. Meanwhile, the old skin of the 

 nymph serves as a sort of boat of which its own body 

 serves as the mast, only a very small portion of the hinder 

 extremity touching it. Next, having drawn from their 

 sheaths the four anterior legs and then the hinder ones, 

 it carries them forward. Soon after it bends towards the 

 water and arranges its limbs, and thus assured of safety, it 

 unfolds its wings and flies off. When first it escapes it 

 is whitish with the thorax greenish, but it very quickly 

 after assumes the proper colours of the adult insect. 



Both larvae and pupae show a remarkable degree of resis- 

 ting power to physical, and even chemical agencies, especi- 

 ally those of Culices, though those of Anopheles appear to 

 bear cold better. In Celli's experiments on them, it was found 



