276 RESPIRATION OF INSECTS AND SPIDERS. 



the extraordinary activity of their respiration. There are 

 no animals which consume so much oxygen, in proportion 

 to their size, as Insects do when they are in motion ( 308) ; 

 but when they are at rest, their respiration falls to the 

 low standard of the tribes to which they bear 

 the greatest general resemblance. Although, as 

 we have seen, the respiration of aquatic larvae is 

 sometimes accomplished by means of gills, yet 

 many aquatic Iarva3 breathe air by means of 

 tracheae ; and such are consequently obliged, like 

 Whales and other aquatic Mammals, to come 

 occasionally to the surface for the purpose of 

 gaining a fresh supply of air. The larva of the 

 Gnat, which breathes in this manner, has one of 

 the spiracles of its tail-segment prolonged into a 

 tube ; and it may often be seen suspended, as it 

 were, in the water, with its head downwards, the 

 end of this tube (t, fig. 160) being at the surface. 



322. In the greater number of perfect Insects, we find the 

 trachese dilated at certain parts into large air-sacs (fig. 159) ; 

 these are usually largest in Insects that sustain the longest 

 and most powerful flight ; in some of which, as in the 

 common Bee, they occupy a greater portion of the trunk than 

 they do in the insect whose system of air-tubes has been just 

 represented, this insect, the Nepa or water-scorpion, being 

 of aquatic habits, and seldom using its wings for flight. 

 There can be little doubt that one use of these cavities is to 

 diminish the specific gravity of the Insect, and thus to render 

 it more buoyant in the atmosphere ; but it would not seem 

 improbable that they are intended to contain a store of air 

 for its use while on the wing, as at that time a part of 

 the spiracles are closed. We shall find in Birds, the Insects 

 of the Vertebrated division, a structure bearing remarkable 

 analogy to this ( 326). 



323. In some of the ARACHNIDA, such as the Cheese-mite, 

 the respiration is accomplished by tracheae, as in Insects ; but 

 in the Spiders it is performed by a different kind of apparatus. 

 Instead of opening into a system of prolonged tubes, each 

 spiracle leads to a little chamber, the lining membrane of 

 which is arranged in a number of folds that lie together like 

 the leaves of a book; and thus a large surface is exposed 



