188 OF INSECTS. 



These observations belong to the whole insect tribe, or 

 to a great part of them. Other observations are limited 

 to fewer species ; but not, perhaps, less important or satis- 

 factory. 



I. The organization in the abdomen of the silk-icorm 

 or spider, whereby these insects form their thread, is as 

 incontestably mechanical, as a wire-drawer's mill. In the 

 body of the silk-worm, are two bags, remarkable for their 

 form, position, and use. (PL XXXIII. fig 1.) They wind 

 round the intestine, when drawn out they are ten inches 

 in length, though the animal itself be only two. Within 

 these bags is collected a glue ; and communicating with 

 the bags, are two paps or outlets, perforated, like a grater, 

 by a number of small holes. The glue or gum, being pass- 

 ed through these minute apertures, forms hairs of almost 

 imperceptible fineness ; and these hairs, when joined, com- 

 pose the silk which we wind off from the cone, in which 

 the silk-worm has wrapped itself up; in the spider the web 

 is formed of this thread. In both cases, the extremity of 

 the thread, by means of its adhesive quality, is first at- 

 tached by the animal to some external hold ; and the end 

 being now fastened to a point, the insect, by turning round 

 its body, or by receding from that point, draws out the 

 thread through the holes above described, by an operation, 

 as hath been observed, exactly similar to the drawing of 

 wire. The thread, like the wire, is form.ed by the hole 

 through which it passes. In one respect there is a dif- 

 ference. The wire is the metal unaltered, except in figure. 

 In the animal process, the nature of the substance is some- 

 what changed, as well as the form ; for, as it exists within 

 the insect, it is a soft, clammy gum or glue. The thread 

 acquires, it is probable, its firmness and tenacity from the 

 action of the air upon its surface, in the moment of expo- 

 sure ; and a thread so fine is almost all surface. This 

 property, however, of the paste, is part of the contrivance. 

 (PI. XXXIII. fig. 2.) 



The mechanism itself consists of the bags, or reser- 

 voirs, into which the glue is collected, and of the external 

 holes communicating with these bags : and the action of 

 the machine is seen, in the forming of a thread, as wire 

 is formed, by forcing the material already prepared, 

 through holes of proper dimensions. The secretion is an 

 act too subtile for our discernment, except as we perceive 

 it by the produce. But one thing answers to another : the 

 secretory glands to the quality and consistence required m 



