io6 NATURAL HISTORY OF AQUATIC INSECTS CH. 



reason can be given why the larval arrangement 

 should not continue. But a time comes when the 

 {]y has to escape from the pupa-case. The skin splits 

 along the back of the thorax, and here the fly emerges, 

 extricating its legs, wings, head, and abdomen from 

 their close-fitting envelopes. The mouth-parts must 

 be drawn backwards out of their larval sheaths, 

 the legs upwards, and the abdomen forwards, so 

 that there is only one possible place of escape — viz., 

 b)' the back of the thorax, where all these lines of 

 movement converge. If, then, the fly must escape by 

 the back of the thorax, the back of the thorax must 

 float uppermost. Otherwise the fly would emerge 

 into the water instead of into the air. Granting that 

 the back of the thorax must float uppermost in the j 

 pupal stage, it is clear that here the respiratory tubes 

 must be set. 



The mouth of the female Gnat is provided with a 

 case of instruments for piercing the skin and drawing 

 blood. The foremost of these is a tube split along its 

 hinder side, which lies in front of the rest, and is used 

 in suction. This, though long and slender, is stouter 

 than the delicate parts behind it, and it serves to 

 stiffen and protect them. Then come five long and 

 slender blades of great delicacy. Two pairs correspond 

 to the mandibles and maxillae of other Insects, though 

 here they are so simplified and attenuated that it 

 is not easy to make out the correspondence. The 

 maxillae are furnished near their tips with a row of 

 extremely minute saw-teeth. There is also a fifth 

 unpaired implement, which is an extraordinary 

 development of a part of the Insect's mouth which is 



