380 PHYSIOLOGY. 



position as in the higher animals, particularly as the entire digestion of 

 insects is almost limited to the imbibition of the juices contained in 

 their food. 



225. 



Lastly, we must here treat of some peculiar secretions which are the 

 produce of digestion, or at least in their fundamental parts, but which 

 exercise no influence upon it : among these we consider the secretion of 

 the spinning vessels and other secerning organs, namely, those of the 

 poison glands. 



The spinning vessels ( 112), which are found only in larvae, are 

 long twisted canals, which empty themselves into the spinning vessel 

 found in the under lip, or in some rare instances, for example, in the 

 larva of Myrmecoleon, present themselves in the shape of a pyriform 

 bag, which, in the perfect insect, appears to be transformed into the 

 colon : they lie at the anal extremity, and contain a viscous thud, which, 

 in the younger larvae, is quite transparent, but, in more mature ones, 

 it is more opaque and thicker. From this fluid the larva spins delicate 

 filaments, which speedily harden in the air, and are then no longer 

 soluble in water. The entire spinning vessel also, when dried in the 

 air, likewise hardens to a firm fragile mass. Chemical analysis discovers 

 the components of this fluid to consist of a substance like lime, a waxy 

 portion, and a little coloured oil which smells like anise. Acids poured 

 upon it harden it ; in young caterpillars it precipitates a flocky sub- 

 stance (albumen) ; but in very concentrated acid it dissolves, as well as 

 in a solution of pure potass : from the former it was precipitated by the 

 addition of water, and from the latter by that of acid in a flocky shape. 

 Hence it appears, that, besides animal albumen, a resinous and an oily 

 substance form components of the spinning fluid, in favour of which 

 the adhesiveness of the fresh material, its rapid drying, and fragility in 

 a mass, speak greatly. It is, consequently, purely an excretion, and is 

 made for the purpose of removing from the body the oily and resinous 

 vegetable portions which are received into the blood by digestion, and 

 again separated from it by the spinning vessels. In the spiders, which 

 feed upon animal substances, and, therefore, doubtlessly, in the larvae 

 of the Phryganea and in the Antlion, &c., which also devour animal 

 matter, it jilso contains ammonia * and a material allied to the horny 



* Gmelin's Chcmic, vol. ii. Pt. '2, p. 1-I7.V 



