NO. 6 CATERPILLAR AND BUTTERFLY — SNODGRASS 27 



tion by the labial glands is common to the larvae of Trichoptera, 

 Lepidoptera, and Hymenoptera, and it may be supposed that the func- 

 tional change in the glands took place in the common ancestors of 

 these three groups. Among the Neuroptera some species secrete silk 

 in the Malpighian tubules and spin it from the anus. The female of 

 the water beetle Hydrophilus pistaceas, described by Lesperon (1937), 

 deposits her eggs on the lower surface of the leaves or stems of water 

 plants enclosed in a cocoon of silk spun from glands opening into the 

 oviducts at the bases of the ovaries. A luminous, silk-spinning 



Lm Ant 



WMMx 



Fig. 14. — The polypod stage of the embryo of Pieris rapae 

 (from Eastham, 1930). 



Ab, abdomen; Ant, antenna; emH, embryonic head; L, thoracic leg; Md, 

 mandible ; iMx, first maxilla ; 2M.V, second maxilla. 



mycetophilid fly larva, Bolitophila luminosa, as described by Ganguly 

 (i960), produces silk in the labial glands and spins it as a snare for 

 capturing insects attracted bv light generated in its Malpighian tubules. 

 The so-called balloon fly, Hilar a of the Empididae, has been shown 

 by Eltringham (1928) to spin its silk from glands in the enlarged 

 basal tarsomeres of the fore legs. The Embioptera are well known 

 to spin silk from glands in the fore tarsi. The art of silk spinning 

 has been most highly cultivated by the spiders, whose silk glands are 

 in the abdomen. 



Lesperon (1937) has described the histology and physiology of the 

 silk glands and the secretion of silk in the several groups of silk- 

 spinning insects. Bradfield (1951) discusses the relation of phos- 

 phatases and nucleic acids in the silk glands of spiders and caterpiMars, 

 showing their function in the secretion and discharge of silk from 

 the glands. 



Of the various ways that caterpillars use their silk, we may note 

 the following. Some species that live in trees but pupate on the 

 ground let themselves down by dropping at the end of a thread drawn 



