328 



INTRODUCTION TO ARTHROPODS 



In mosquitoes (Fig. 189) the salivary glands consist of three 

 lobes, one lobe being noticeably different in appearance and 

 secretion from the others. The pharynx connects with the 

 stomach by a slender oesophagus. Various means are used by 



blood-sucking insects to increase 

 their capacity. In the bugs 

 (Fig. 135) the stomach is ex- 

 tremely distensible and serves as 

 a storage reservoir. In fleas and 

 many biting flies there is an ex- 

 pansion of the oesophagus an- 

 terior to the true stomach, called 

 the proventriculus ; in mosqui- 

 toes there are capacious pouch- 

 like food reservoirs or outgrowths 

 from the oesophagus in addition 

 to the proventriculus (Fig. 189). 

 Just behind the true stomach at 

 the beginning of the intestine 

 there open a number of long 

 slender tubes, the " Malpighian 

 tubules" (Fig. 135, malp. t.). 

 These are the excretory organs, 



FIG. 135. Digestive tract of a Re- 



duviid bug; ace. sal. gi., accessory corresponding to the kidneys of 

 salivary gland; conn, d., connecting ver t e brate animals. Their func- 



tion is to collect the waste 

 matter of metabolism from the 

 blood and pour it into the in- 

 testine, whence it can ultimately be voided through the anus. 

 The length of the intestine varies, being usually longer in vege- 

 table-feeding insects than in carnivorous ones. It often has a 

 marked expansion, the anal pouch, at its posterior end. 



The tracheae of insects, as already intimated, are really a ven- 

 tilation system consisting of air tubes ramifying all through the 

 body even to the tips of the antennae and legs. They open by a 

 series of pores along the sides of the insect known as spiracles, 

 which function as do the nostrils of higher animals. The prin- 

 ciple of oil sprays for insects is to form a film of oil over the 

 spiracles, so that the insects will suffocate. 

 The nervous system of insects is very highly developed for 



---roa!p.t. 



-rrct. 



duct between salivary glands; int., in- 

 testine; malp. t., malpighian tubules; 

 oes., oesophagus; rect., rectum; sal. gl., 

 salivary gland. (Partly after Dufour.) 



