262 PHYSIOLOGY OF THE INVERTEBRATA. 



The larva of Dicranura fiorcula does not eject an irritant 

 secretion, but it possesses an eversible "gland" as a defensive 

 organ. A similar structure is present in the larva of B. vinula, 

 but it is unable to evert its prothoracic " gland " voluntarily. 

 This structure is eversible in the larvse of Melitcea artemis, 

 Orgyia pudibtiTida, Orgyia antiqua, and Liparis aiiriflua ; and 

 there is no doubt that these defensive structures are of con- 

 stant occurrence in Lepidopterous larvae. 



The power of everting the " gland " in the larva of D. vinula, 

 has been lost, due to the fact that the " larva has acquired the 

 remarkable power of ejecting the intensely irritant secretion 

 to a considerable distance by forcing it through the narrow- 

 chink, with its closely approximated lips, which constitutes 

 the mouth of the duct leading to the sac. Such a formidable 

 means of defence may readily have supplemented the more 

 usual method of eversion, a method which can only give rise 

 to the discharge of vapour into the air, instead of a well- 

 directed stream of fluid, which, if volatile, as it is in these 

 larvae, of course produces abundance of vapour." 



The eversible glands of the larva of Liparis auriflua are 

 not often completely everted, but they are very sensitive to 

 tactile impressions, and on " stimulation a clear, transparent 

 secretion appears in the lumen, being probably raised by 

 partial eversion. The secretion is not acid to litmus paper, 

 but it possesses a peculiar and penetrating odour." 



The ejection of defensive fluids and vapours are not con- 

 fined to the anterior parts of insects, for in the Bombardier 

 Beetles, according to Dr. L^on Dufour, a pungent vapour, 

 resembling nitric acid in its properties, is ejected from the 

 anus. Brachinus displosor will furnish twelve such discharges, 

 but subsequently explosion with noise is replaced by the 

 emission of a yellowish or brownish fluid, which readily 

 vaporises. These discharges are meant to arrest the onset of 

 larger predacious beetles. Brachinus crepitans is sometimes 

 gregarious, and when one individual is disturbed the whole 



