230 INSECTS. 



structure is necessary for the accommodation of the animal to its 

 changes of food ; but where the larvae and the perfect insect feed 

 on the same materials this alteration in organization does not 

 take place. 



The anus, the chief excretory organ, is the inferior or rather 

 posterior opening of the intestinal canal. It terminates in a kind 

 of cloaca, in which are also found the orifices of the organs of 

 reproduction. There are neither kidneys nor bladder in insects ; 

 and no organs representing the pancreas and conglomerated 

 glands of the higher classes have ever been discovered. The 

 substitute for the liver is a tuft of floating filaments, which sur- 

 round the greater portion of the intestinal canal, and which 

 take their rise towards the third of its length, on the side of the 

 stomach. There are also no organs equivalent to the salivary 

 glands of the higher classes ; but a fluid analogous to saliva, or 

 biliary matter of a blackish colour and caustic quality, is secreted 

 by floating vessels. The saliva of the Scaraboci is of a brown 

 colour, very acrid, and introduced into a wound produces inflam- 

 mation. A similar fluid is perhaps injected by the Culices into 

 their bite. 



Besides the secretory organs proper to nutrition and genera- 

 tion, others are found in certain insects for secreting fluids, either 

 calculated for defence, or for protecting them from variations of 

 temperature during their transformations. The acrid and fetid 

 fluids with which some insects defend themselves are produced 

 by small tortuous tubes, and accumulated in two vesicles near 

 the anus. The Carabi and the Dytisci secrete acid fluids 

 which redden vegetable blue ; the Brachini discharge an acrid 

 vapour, which gives considerable pain ; a species of Blaps pro- 

 duces a brown fetid oil, which swims upon water ; the silk- 

 worm possesses organs for secreting the silky matter of which 

 the threads of silk are formed ; and in the Hymenoptera, such as 

 wasps, bees, sphexides, &c. the extremity of the abdomen incloses 

 a sting, calculated for attack or defence. This sting is a hollow 

 canal furnished with muscles, of which the contraction or dilata- 

 tion projects or withdraws it at the will of the animal. At the 

 base of this hollow tube is found a gland which secretes the acrid 

 or poisonous fluid. 



The sexes in insects are always in separate individuals, male 

 and female, and coupling takes place at certain seasons, as in the 



