THE INTERNAL ANATOMY OF INSECTS 



99 



qanal, 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 

 Fig. 113. Molting-fluid glands desiccation, Tower ('06) states that he 

 of the last larval instar of has never f oum } these glands in larva? 

 Leptinotarsadectml-meata,just . . 



b)fore pupation; le, larval that live in burro ws, or in the soil, or in 

 epidermis; Id, larval dermis; cells; i n these cases the molting fluid is 

 mf, molting fluid; pe, forming 



pupal epidermis; h, hypoder- apparently secreted by the entire hypo- 

 mis; g molting fluid gland dermal layer. 

 (After Tower). 



Glands connected with setae. There 



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

 is through the lumen 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 aeta. 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. 1 14. Glandular s? ta; s, seta; 

 c, cuticula; h, hypodermis; bm, 

 basement membrane; tr, tricho- 

 gen; g, gland (After Holmgren). 



