ANATOMY AND PHYSIOLOGY 



107 



present and the glands are frequently branched or lobed, and though 

 usually confined to the head, may extend into the thorax or even into 

 the abdomen. 



Many insects have more than one pair of glands opening into the 

 pharynx or oesophagus; thus the honey bee has six pairs and Hymenop- 



FIG. 155. Right salivary gland of cockroach, ventral 

 aspect, c, common duct; g, gland; h, hypopharynx; r, 

 reservoir. After Mi ALL and DENNY. 



FIG. 156. Histology of 

 salivary gland of Cacilius, 

 radial section. 6, basement 

 membrane; c, canal; g, g, 

 glandular cell; i, intima; w, 

 nucleus. After KOLBE. 



tera as a whole have as many as ten different pairs. Though all these 

 are loosely spoken of as salivary glands, it is better to restrict that 

 term to the pair of glands that open at the hypopharynx. 



All these cephalic glands are evaginations of the stomodaeum (ecto- 

 dermal in origin) and consist of an epithelial layer with the customary 

 intima and basement membrane (Fig. 156). The nuclei are large, as is 

 usually the case in glandular cells, and 

 the cytoplasm consists of a dense 

 framework (appearing in sections as a 

 network) enclosing vacuoles of a clear 



;nh^tanrp thp^prrptinrr the rhitinons FlG ' I 57- One of the three-lobed 



3 salivary glands of a mosquito. The 



intima is penetrated by fine pore canals middle lobe secretes the poison. After 



.-, T 1-1,1 . MACLOSKIE, from the American Nat- 



through which the secretion passes. ura n st . 



In many insects, notably the cockroach, 



the common duct is held distended by spiral threads which give the duct 



much the appearance of a trachea. 



In herbivorous insects the saliva changes starch into glucose, as in 

 vertebrates; in carnivorous forms it acts on proteids and is often used 

 to poison the prey, as in the larva of Dytiscus. In the mosquito each 

 gland is three-lobed (Fig. 157) ; the middle lobe is different in appearance 

 from the two others and secretes a poisonous fluid 'which is carried out 



