THE INTERNAL ANATOMY OF INSECTS 



99 



-fe.. ,,.... . i:..... J -—Id 



■■--■ . -'- — ■ -•;"*--'.'-±.'f^. _ ^y 



Fig. 113. — Moltinpj-fluid glands 

 of the last larval instar of 

 Lept molar sa decimlineata, just 

 before pupation; le, larval 

 epidermis; Id, larval dermis; 

 mf, molting fluid; pe, forming 

 pupal epidermis; h, hypoder- 

 mis; g, molting fluid gland 

 (After Tower). 



canal, and the glands of the reproductive organs. In this place 

 reference is made only to the hypodermal glands, those developed 



from the hypodermis. 



The Molting-fluid glands. — Under this 

 head are classed those unicellular, hypo- 

 dermal glands that secrete a fluid that 

 facilitates the process of molting, as des- 

 cribed in the next chapter (Fig. 113). 



While molting-fluid glands are very 

 numerous and conspicuous in certain 

 insects, those living freely exposed where 

 there exists the greatest liability to rapid 

 desiccation, Tower ('06) states that he 

 has never found these glands in larvae 

 that live in burrows, or in the soil, or in 

 cells; in these cases the molting fluid is 

 apparently secreted by the entire hypo- 

 dermal layer. 



Glands connected with setae. — There 



are in insects several kinds of glands in which the outlet of the gland 



is through the Itmien of a seta. The function of the excretions of 



these glands is various as indicated 



below. There are also differences in 



the manner of issuance of the excre- 

 tion from the seta. In some cases, as 



in the tenent hairs on the feet of certain 



insects, the excretion can be seen to 



issue through a pore at the tip of the 



seta. In some kinds of venomous setae 



the tip of the seta breaks off in the 



wound made by it and thus sets free 



the venom. But in most cases the 



manner of issuance has not been deter- 

 mined, although it is commonly believed 



to be by means of a minute pore or 



pores in the seta, the thickness of the 



wall of the seta making it improbable 



that the excretion passes from the seta 



by osmosis. 



The structure of a glandular seta 



is illustrated by Figure 114; the 



essential difference between such a seta and an ordinary one, that is a 



Fig. 114. — Glandular s^ta; 5, seta; 

 c, cuticula; h, hypodermis; bm, 

 basement membrane; <r, tricho- 

 gen; g, gland (After Holmgren). 



