THE INTERNAL ANATOMY OF INSECTS 



101 



secrete an adhesive fluid; this enables the insect to walk on the lower 

 surface of objects (Fig. ii6). 



Fig. 1 16. — A, terminal part of a tenent hair from Eupolus, showing canal in the 

 hair and opening near the tip; B, cross-section through a tarsal segment of 

 Telephorus; c, cuticula; g, gland of tenent hair; h, h, tactile hairs; hy, hvpo- 

 derrais; n, nerve; s, sense-cell of tactile hair; t, t, tenent hairs (After Dewitz). 



The osmeteria. — In many insects there are hypodermal glands that 

 open into sac-like invaginations of the body-wall which can be 

 evaginated when the insect wishes to make use of the secretion pro- 

 duced by these glands; such an organ iS termed an osmeterium. The 

 invagination of the osmeterium admits of an accumulation of the 

 products of the gland within the cavity of the sac thus formed; when 

 the osmeteritm^ is evaginated the secretion becomes exposed to the air, 

 being then on. the outside of the osmeterium, and rapid diffusion, of 

 the secretion results. 



The most familiar examples of osmeteria are those of the larvas 

 of the swallow-tailed butterflies, which are forked, and are thrust out 

 from the upper part of the prothorax when the caterpillar is disturbed, 



and which 

 diffuse a dis- 

 agreeable odor 

 (Fig. 117). 

 They are ob- 

 viously organs 

 of defense. 

 Osmeteria 



are present in the larvse of certain blue butterflies, Lycasnidse. These 

 are in the seventh and eighth abdominal segments, and secrete a 

 honey-dew, which attracts ants that attend and probably protect 

 the larvae. The osmeteria of many other caterpillars have been 

 described. 



Fig. 117 — Larva of Papilio thoas; 0, osmeterium expanded. 



