384 PHYSIOLOGY. 



betrays itself not merely by its smell but more by a peculiar but not 

 unpleasant taste. That this acid is found especially in the abdomen is 

 well known, but we are unacquainted with the organ that secretes it ; 

 it is probable that the poison organs and the acid are both merely a very 

 sharp urine. 



Among the Lepidoptera peculiar secreting organs have been found 

 in some larvae, for instance, in the larva of Harpya vinula, which has 

 a little bag at the ventral plate of the first abdominal segment, that, 

 when filled, is of about the size of a pea, and the aperture to which is 

 a transverse incision at the same spot. The fluid contained in it is a 

 powerful acid, which produces pain and inflammation upon a delicate 

 skin*. In the caterpillar of Pieris Mackaon there is a similar furcate 

 secreting organ in the neck, which is projected upon its being roughly 

 handled. The getting greasy, as it is called in Lepidoptera, also indicates 

 a great provision of secreted juices. In Harpya vinula it is frequently 

 the case, and we might thence suppose it to be consequent upon 

 the secretions of the caterpillar. The liquid, however, seems to be 

 no oil, but rather an acid. Lastly, among the Dipt era we find 

 individual instances of a presence of peculiar secretions, for example, 

 in Ccenomya ferrvginca, Meig. (Sicus ferrug., S. bilicor, and S. 

 errans, Fab.) ; some of the flies which belong to the division of those 

 with a spiny scutellum (Dipt, notacantha), which Meigen called whey 

 flies, from their penetrating smell, resembling that of green whey 

 cheese. This smell, which proceeds from the whole body, and which 

 cannot be ascribed to any local excretion, remains even a long time 

 after death, whereas the majority of such odours then speedily eva- 

 porate. 



226. 



II. FUNCTION OF THE AIR TUBES, RESPIRATION. 



The chief object of respiration is to adapt the circulating fluid 

 destined for assimilation with the organic mass to that purpose, by the 

 addition of another substance, viz., atmospheric air or oxygen. To 

 attain this we find in the majority of instances distinct respiratory 

 organs, namely, a more or less distributed respiratory surface, which 

 must be purely considered as either an internally or externally produced 

 continuation of the epidermis, and in which the fluid circulates, and 



* Rengger's Physiolog. Untcrsuch., p. 427. 



