STRUCTURE AND CLASSIFICATfON. 



3» 



I'IG. 15. 



modified for special purposes ; as in some of the piercing insects 

 like bugs or the mosquitoes, where an irritant poison is secreted, 

 or in certain beetles, where the saliva has 

 both a staining and a burning effect upon 

 the skin, or in many larvae, where the se- 

 cretion forms a silk when hardened by ex- 

 posure to the air. The anal glands may 

 be similarly modified, but much more 

 rarely than those at the anterior end of the 

 body. In the "Bombardier beetle" their 

 secretion volatilizes suddenly when expelled 

 into the air, and forms a blue smoke, like 

 the discharge from a small cannon. In 

 Hymenoptera, including bees and wasps, 

 the secretion is poisonous, and accessory 

 to the functions of the sting. More rarely 

 a silk is produced from these glands. Taken 

 altogether, the digestive system varies 

 greatly in length, being sometimes a 

 straight tube only, the various portions 

 very incompletely divided off. In other 

 cases it is quite complex, coiled upon 

 itself, the parts well defined, and the sys- 

 tem as a whole two or three times the 

 length of the entire insect, different kinds 

 of structure being found in the various 

 parts. The figures given herewith illustrate 



some of these differences. Salivary gland of the cat- 



-. , , . 1 11 erpillar of Melitha ceto. 



In the sucking or haustellate type the modified for silk spinning: 

 essential structure of the alimentary canal mandible of the same, 

 and the differentiation of the parts are much 



the same. There is, however, a great reduction in the size of the 

 gizzard, which is very often entirely wanting. If present, the 

 armature is reduced to a mere ridging, or a slight, irregular 

 thickening of the walls of the posterior end of the crop. In the 

 Lepidoptera there is often a peculiar accessory pouch, which 

 seems to serve the purpose of a storage reservoir, or secondary 

 crop, and this is attached by a narrow neck to the opening of the 

 true crop. In the bees the crop is unusually elastic, and capable 



