Hahits of Evaniii Desjardinsii, Blanch. 475 



which the final result interested me intensely. In spite of 

 this, I am convinced that the Evania intended to introduce 

 an egg into the ovarian capsule still carried by the Blatta. 



An author quoted by Westwood [loc. cit.) — Mr. Stephens 

 — seems disposed to admit that each species of the genu3 

 Evania is parasitic on a single species of Blatta, whilst the 

 converse would not be true — that is to say, that a single 

 species of Blatta would have as parasites two species of the 

 genus Evania. Relying on the fact that E. niinuta and 

 E.fulvipes are abundant in the parts of Europe inhabited by 

 Blatta lapponica, he regards them as indigenous, whilst, 

 according to him, the typical species E. appendig aster, which 

 frequents B. {Periplaneta) orientalis, disseminated by com- 

 merce throughout the world, ought to be considered as 

 imported with this Orthopter. 



If in so concluding Stephens had the intention of formu- 

 lating a general rule, the example of E. Desjardinsii doubly 

 weakens it, because (1) this Evania is parasitic on at 

 least two different species of the genus Blatta, (2 ) neither of 

 these species is peculiar to the Mascareignes. We ought to 

 conclude, following Stephens, that E. Desjardinsii is tropical 

 American in origin, because B. americana is such, or that it 

 comes from the same region as B. maderce — two inadmissible 

 suppositions, since the Evania in question is peculiar to the 

 Mascareigne Islands. It is, however, right to recognize that, 

 if this rule is actually at fault, it is in origin partly correct. 

 In fact before the introduction of B. americana and B. ma- 

 derce, E. Desjardinsii ought to have been the parasite of one 

 or more Blattm belonging specially to the fauna of the Mas- 

 careigne Islands. Perhaps even this Evania is still parasitic 

 on this species — or these indigenous species, among which I 

 may mention Blatta corticum and B. cinerea — which measures 

 as much as 28 millim., and whose ootheca ought to be of 

 considerable size. 



Actually, and in a general way, it appears rational to look 

 rather for a relation between the figure of the Evania and the 

 ootheca of the corresponding Blatta. The larva of the 

 Hymenopter must find in the ovarian capsule space for 

 development, and, above all, a sufficiency of food. We can 

 understand that, the second condition being fulfilled, the first 

 is not absolutely indispensable, because in the case in which, 

 at a given moment, the dimensions of the ootheca become 

 insufficient, the larva could escape from its asylum, weave 

 a cocoon, and then complete its metamorphosis. 



There remain several important points in the habits and 

 development of Evania Desjardinsii to be worked out ; I 

 hope to be able to do this shortly. 



