Respiration of Insects, 323 



spiracles are placed at the foot of the raised ridge on either side. 

 From the top a horizontal line of hairs stretches towards a 

 similar row from the raised median ridge, thus forming two air- 

 chambers to supply the stigmata, and permit a prolonged stay 

 under water. In this case the spiracles are really ventral, 

 though, as the creature always swims on its back, they are prac- 

 tically on the upper surface. 



A similar result is produced among many aquatic Coleoptera 

 by the cavity formed by the elytra, which can often be seen 

 being replenished as the creature brings the tip of its abdomen 

 to the surface. 



In the Hydrophili we find part of the under side clothed with 

 pubescence, which retains the air, thus rendering it somewhat 

 difficult for them to get to the bottom of the water ; H. piceus 

 in this group gathers in the air with its antennae, bringing its 

 head to the surface for this purpose. 



To guard against the entrance of dirt or foreign matters into 

 the tracheae, the spiracles are protected by hairs, spiculse, 

 ridges, or sieve-like coverings, of which I show four examples. 



A curious modification of spiracle can be seen in Xe/ja, the 

 familiar water-scorpion. Here it is anal, and composed of a 

 long projecting tube; but, as great danger would arise from its 

 getting stopped up. Nature has provided that it shall be made in 

 two halves, with the sides fringed with hairs, which interlock ; 

 they can then be readily separated, and any impurities removed. 

 In Eiistalis a long tube is provided, but in this case it is telescopic, 

 being capable of protrusion for a considerable length. 



The muscular apparatus by which spiracles can be closed is 

 composed of four parts — the bow, the lip, the lever, and the 

 muscle ; and is thus described by Huxley : — " The bow is a 

 thickening of one-half of the circumference of the chitinous 

 lining. The Up is formed by the other half of the circumference, 

 and the lever is a chitinous process connected with one end of 

 the bow or with the lip. When the lever is single the muscle, 

 which is attached to it, passes over the lip, and is inserted into 

 the opposite end of the bow. When it contracts it therefore 

 presses the lip against the bow. When two levers are present 

 they are attached to opposite ends of the lip and bow, and the 

 muscle extends between their extremities. The efiect of its 

 contraction is to thrust the free edge of the lip against the bow." 



That something more is necessary than the mere presence of 

 air in the tracheae is evident, and it is to the largely developed 

 muscular system we must look for a means of circulation. When 

 a wasp alights on a fence you must all have observed the con- 

 traction and expansion of the abdomen. With each pulsation 

 the process of expiration and inspiration is effected, and it will 

 be readily understood that if the spiracles were closed after an 



