spinners of Silk. 



305 



Another simple, but highly useful example of the spinning power is exhibited 

 by the leaf-rolling caterpillars and the elongated caterpillars known as geometers 

 (with which we deal elsewhere) from their peculiar manner of progression, in which 

 they appear to be carefully measuring the distance traversed. Some caterpillars 

 of these two families, when the bough upon which they are feeding is rudeh' jerked, 

 at once loose their hold, but simultaneously spin a single thread by which they 

 hang suspended in mid-air until the supposed danger appears to have passed, when 

 they ascend the thread and regain their former station. 



As a rule the spinning of butterfly-caterpillars is restricted to the fabrication 

 of a silken pad into which the terminal hooks of the chrysalis become attached, 

 and of a girdle around what we may term the waist of the chrysalis. The fluid 

 silk is produced by two large glands, one on each side of the body, whose ducts 

 unite and are continued externally as the spinneret, which is a point on the middle 

 line of the lip, differently developed in the different families and species. The 

 glands are of simple structure and vary in size according to the amount of silk- 

 production required by the species. In some of the moth-caterpillars that elaborate 

 thick cocoons their length and weight are considerable : the silkworm, for instance, 

 possessing a pair of silk-glands each measuring five times the full length of the 

 body. This length is exceeded by some other species, the caterpillar of the 

 polyphcmus silk-moth,^ for example, having silk-glands seven times the length 

 of its body. In those that do not spin cocoons, on the other hand, the silk-glands 



Photo by] 



The Demoiselle Dragon-Fly. 



The demoiselles are distinct among the slender-bodied dragon-flies by reason of their dark purple colour, 

 glossy with metallic reflections. The photograph shows the characteristic resting attitude o£ this sf>ecies. 



1 Telea polyphcmus. 



[E. Step, F.L.S. 

 The cylindrical body is 



