Introduction to Ani?nal Morphology. 383 



CHAPTER XLVI. 



CLASS 4. — INSECTA [Latreille). 



Arthropods segmented into head,* thorax, and ab- 

 domen ; the first, like that of a myriopod, of six fused 

 somites ; the thorax consists of three, pro-, meso-, and 

 meta-thorax, each of which bears a pair of legs ; the 

 abdomen is of 7-1 1, usually nine somites, f limbless in 

 the adult. Respiration is tracheal. Life is usually 

 limited to one period of sexual maturity, and develop- 

 ment is generally attended with metamorphosis. 



The adult integument is usually chitinous, with 

 hairs, bristles, or scales (calcified in the larva of Stra- 

 tiomys), often with unicellular glands opening into 

 the cuticular pore-canals, or into hairs whose tips 

 must be broken before the secretion is emptied, or 

 into the intermetameric spaces. In some insects are 

 clustered glands secreting wax. In Aphides and 

 those Coccida that secrete wax-threads, they occupy 

 large surface-areas. In bees these glands, opening 

 by many ducts, masses of gland-cells, tracheae, and a 

 covering layer of fat, make up, on front of the ventral 

 rings, the " wax-organ." Aphides have also surface 

 honey-glands. Some dermal gland-cells are phos- 

 phorescent, and odoriferous glands are often seated at 

 the bases of the limbs, or intermetamerically, or around 

 the anus. The products of these are often emptied 



* Some Coccina have the prothorax and head united. 



t Eleven, the normal number, only present in Orthoptera. Tvi^o are 

 most commonly aborted or indistinct. Of these, strictly, eight are abdo- 

 minal and three post-abdominal. 



