488 THE INVERTEBRATA 



nearly always absent and the labium is represented by a transverse 

 plate and a pair of three-jointed palps. 



Each half of the proboscis is a tube in itself into which passes blood 

 from the head, and also a trachea and a nerve. Across the cavity of 

 this tube there pass a number of diagonal muscles, the contraction of 

 which causes the whole organ to roll up into its characteristic position 

 beneath the head and thorax (Fig. 339). How the proboscis is ex- 

 tended is not fully understood; in all probability, blood pressure 

 plays an important part. 



The length of the proboscis in many cases corresponds to the depth 

 of the corolla of the flower which the species frequents, and in the 

 Sphingidae (hawkmoths) is greater than that of the body. Sometimes 



Fig* 338. A, Tryphaetia pronuba, with venation and frenulum (Jr.); ^ con- 

 dition on right. Original. B, Obtect pupa of Platyhedra gossypiella. After 

 Metcalf and Flint. 



the organ is reduced or absent and the animal does not then feed 

 in the adult state at all. 



The beginnings of the proboscis can be traced in primitive forms. 

 In the Micropterygidae there are biting mandibles and maxillae of 

 the type usually found in insects which masticate their food: in 

 Micropteryx there is no proboscis, the animal feeding on pollen; in 

 Eriocrania the mandibles are non-dentate, the laciniae are lost and 

 the galeae form a short proboscis. 



The characteristic feature of the wings is the clothing of scales 

 (Fig. 338). These latter are formed by enlarged hypodermal cells, 

 and their main function appears to be the presentation of colour due 

 either to striation of the surface causing interference colours, or in 

 lesser degree to the pigment they contain (like the uric acid of the 

 Pieridae). There also occur "scent scales" which may have a sexual 



