328 INSECT AKCHITECTURE. 



not larger than the egg of the gold-crested wren. It 

 has been considered by Brahm a puzzling circum- 

 stance, that this cocoon is usually perforated with 

 one or two little holes, as if made by a pin from with- 

 out ; and Kirby and Spence tell us that their use 

 has not been ascertained.* May they not be left as 

 air-holes for the included chrysalis, as the close texture 

 of the cocoon might, without this provision, prove 

 fatal to the animal f Yet, on comparing one of these 

 with a similar cocoon of the large egger-moth (La- 

 siocampa Quercus), we find no air-holes in the latter, 

 as we might have been led to expect from the close- 

 ness of its texture. We found a cocoon of a saw-fly 

 (Trickiosoma), about the same size as that of the 

 egger, attached to a hawthorn twig, in a hedge at 

 New Cross, Deptford, but of a leathery texture, and, 

 externally, exactly the colour of the bark of the 

 tree. During the summer of 1 830 we found a con- 

 siderable number of the same cocoons. These were 

 all without air-holes. The egger, we may remark, 

 unlike the dock-weevil, or the bee-grub just men- 

 tioned, can work her cocoon without any point of 

 attachment. We had a colony of these caterpillars in 

 the summer of 1825, brought from Epping Forest, 

 and saw several of them work their cocoons, and we 

 could not but admire the dexterity with which they 

 avoided filling up the little pin-holes. The supply of 

 their building material was evidently measured out to 

 them in the exact quantity required ; for when we 

 broke down a portion of their wall, by way of expe- 

 riment, they did not make it above half the thickness 

 of the previous portion, though they plainly preferred 

 having a thin wall to leaving the breach unclosed.t 

 Several species of caterpillars that spin only silk 

 are social, like some of those we formerly mentioned, 

 * Brahm's Ins. Nat. 289 ; and Kirby and Spence's Intr. iii. 223. 

 t J. R. 



