PHYSIOLOGY OF THE INVERTEBRATA. 261 



The larva of Dicraniira, vimUa possesses a gland which 

 secretes formic acid. The duct of this gland opens in a 

 horizontal slit on the red margin below the true head, and is 

 thus placed in such a position that its contents are ejected 

 in an antei'ior direction. Disturbance causes the larva to 

 withdraw its head still further, and to inflate the red margin, 

 especially in the region of the gland duct, and at the same 

 time the head is always turned in the direction of the dis- 

 turbance. Thus the fluid is thrown towards the cause of the 

 irritation, and the terrifying appearance of the larval full- 

 face is also brought to bear upon it (Poulton). The acid 

 ejected by the larva of D. vinula is a defensive fluid, and no 

 doubt is a means of protection against enemies. 



This defensive fluid is ejected from a transversely placed 

 aperture on the ventral surface of the prothorax, immediately 

 below the head. Mr. E. B. Poulton, F.K.S., Prof. R. Mel- 

 dola, F.R.S., and Prof. W. R. Dunstan have proved by 

 chemical tests that this fluid secreted by the larva of D. 

 vinula is formic acid. " The smell is also quite characteristic, 

 and affords an indication of the large proportion of acid 

 present in the secretion. It is also an interesting fact that 

 the freshly-made and moist cocoon of D. vimda is powerfully 

 acid to test-paper." 



The secretion consists of a pure aqueous solution of formic 

 acid, containing an average of ^;^ per cent, of anhydrous acid. 

 A mature larva will eject 0.05 gramme of the secretion, con- 

 taining 40 per cent, of acid. The rate of secretion is slow ; 

 starvation lessens its amount and decreases the quantity of 

 acid; but there is no difference in the nature of the acid 

 when the larva is fed on poplar instead of willow.'* 



" The larva appears to depend entirely upon tactile stimuli 

 for the direction in which to move its terrifying full-face, 

 and towards which to eject the irritant acid secretion. A isual 

 sensations appear to play no part as guides in the assumption 

 of the defensive attitude." 



* See Report of British Association, 1S87, p. 765, 



