Glands 69 



tnbes. The ducts from the two glands join to form 

 a single tube, which runs into the large duct formed 

 by the union of those from the reservoirs. These 

 reservoirs are large pear-shaped sacs with transparent 

 chitinous walls (l, 42). Spittle glands are very highly 

 developed in the Bees, some of which have as many as 

 eight pairs, seven small pairs in the head and a large 

 pair in the thorax (fig. 47, Nos. I, 2, ^). 



Scent, Poison and other Glands. — It is well 

 known that Cockroaches give otF a disagreeable smell. 

 This is due to the secretion from a pair of glands 

 opening on the back of the hind-body — between the 

 fifth and sixth tergites. Each gland is a pouch 

 formed by inpushing of the skin, lined with chitin 

 and bearing in its cavity manv stitf, branched bristles. 

 Surrounding the chitinous lining is the cellular layer 

 which secretes the evil-smelling fluid. Glands, some- 

 what similar to these, are found on various parts ot 

 the bodies of many insects, and their secretion is of 

 use as a defence against enemies ; certain Butterflies 

 possess such glands at the hinder end of the abdomen 

 (44). But in other insects the substance secreted is 

 sweet-smelling, and is believed to act as an attrac- 

 tion to the opposite sex ; some male Butterflies have 

 patches on the wings which produce sweet scents, 

 others possess fragrant tufts of hairs on the legs (45)« 



Certain insects discharge fluids not merely un- 

 pleasant, but actuallv poisonous. A lobe ot each 

 spittle-gland in Gnats is specially modified for the 

 secretion of the poison to which the well-known 

 painful eflects of the bite of those insects is due. A 

 pair ot poison-glands are present in the head of some 

 Water-bugs ; these open at the base of the beak, and 

 their secretion serves to kill the small insects on which 

 the bug preys. Best known of all poison glands are 

 those connected with the stings ot Wasps and Bees. 



