146 ANATOMY. 



Where we meet with salivary vessels we generally find two ; some 

 insects have, on the contrary, four, each pair of which unite into one 

 evacuating duct (Apis, Cimex, Pulex) ; Nepa has even six salivary 

 vessels, three on each side, all of which open into the cavity of the 

 mouth ; two unite on each side into one stem, the third, which has 

 been considered as a poison-secreting organ, remains separated as far 

 as the mouth. 



Many larvae, particularly the caterpillars of the Lepidoptera, have 

 also four salivary vessels of diiferent structure ; two are slender, very 

 long (Cossus), and filiform ; two broader, sometimes bag-shaped (for 

 example, Cossus ligniperda, O.), and considerably shorter. The first 

 secrete a viscous liquid, from which the caterpillar spins its silk. The 

 evacuating ducts of both unite into one, and open into the under lip, 

 namely, into the canal of the above ( 54) described spinneret. This 

 pipe would therefore be more correctly called spinning vessel. Such 

 spinning vessels are naturally found only in those larvae which prepare 

 a web for their pupa change, such as the caterpillars of the nocturnal 

 Lepidoptera, the larvae of the saw-flies, and of the Phryganodea. It 

 distinguishes itself chiefly by its length and size from the true salivary 

 vessels, which are often very small and insignificant. The true salivary 

 vessels, according to Suckow *, open at the base of the upper mandible 

 with a small warty protuberance (PI. XXI. f. 13), and remain even in 

 the perfected moth ; whereas the spinning vessels totally disappear 

 during the pupa state f . 



In Myrmccoleon the spinning vessels lie at the anal end of the 

 abdomen, and true salivary vessels have not yet been observed in it +. 



The structure of this organ appears, according to all investigations 

 hitherto instituted, to be very variable, for sometimes there are two 

 membranes (the muscular and mucous) and sometimes but one. The 

 former vary in consistency, but occasionally are uniform with those of 

 the intestine ; in the latter case they are transparent and delicate, and 

 occasionally granulated or irregular. 



The length also of the salivary vessels differs much : in some cater- 

 pillars they are two or three times as long as the intestine ; in perfect 

 insects, on the contrary, they are generally shorter, and do not usually 



* Suckow's Physiol. Unternich. uber Insecten und Krustenthiere, p. 28. PI. VII. f. 32. a. 

 f Ib.p. 29.P1. II. f. 1 10. h. h. 

 + Ramdohr, PI. XVII. f. 1 4. 



