XIII CONTRI\'ANCES OF AQUATIC INSECTS 385 



necessary. Some Dragon-fly larvae swim by striking 

 the water with the long abdomen and its terminal 

 plates ; others make sudden rushes b\- the violent 

 expulsion of water from the intestine. Slow larvje 

 creep by legs, hooked tubcrcules, backward-pointing 

 hairs, and the like. Not a few pupae, both aquatic 

 and terrestrial, bear spiny rings which aid in creeping, 

 and enable them to make their way to the air when 

 the fly is about to emerge. 



Or we may review the contrivances which enable 

 aquatic Insects to capture their food. The perforated 

 suctorial mandibles and the mouth-lock of the 

 Dytiscus larva, the mask of the Dragon-fly larva, the 

 " weel " which entangles the struggling victims of the 

 Phantom larva (Corethra), the clasp-knife mechanism 

 of the prehensile fore legs of the Water Scorpion, the 

 snare of the Plectrocnemia larva, the ciliary organs of 

 many aquatic Dipterous larvae, and the pharyngeal 

 strainer of the Eristalis larva are varied in design, 

 perfect in execution. But our admiration for these 

 wonderful implements is mingled with another feeling. 

 The weapons of carnivorous Insects are a little too 

 like a collection of instruments of torture. 



The respiration of aquatic Insects may be alto- 

 gether like that of the terrestrial species, except that 

 the gaseous air to be taken into the body is not so 

 1 cadily procured. We have seen how the wing-covers, 

 the felted back, and the dorsally set spiracles of the 

 winged Dytiscus combine to secure an ample supply 

 of air every time that the Insect floats or swims to 

 the surface. Hydrophilus and Hydrobius employ 

 their peculiar antennae in breathing ; Notonecta its 



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