164 Mr. E. Shelfovd on new 



means evident at first siglit. A knowledge of the habits of 

 an animal should invariably precede all suggestions as to the 

 value of any details of its structure, and I feel convinced that 

 much of the mystery surrounding variations in structure 

 which are spoken of as being merely of importance to the 

 systematic naturalist will be dispelled as our knowledge of 

 the life-histories of the animals exhibiting them increases, A 

 clue to the use of tlie variations in pronotal structure of the 

 cockroaches under notice is afforded by the observations, 

 slight and incomplete though they are, on their habits. The 

 vast majority of Blattidae are insects of cryptic habits, 

 spending most of their life hidden under stones or logs, in 

 decaying vegetation, burrowing in rotten wood, and so forth, 

 and the majority of species are flattened depressed insects. 

 The species of Pilema, on the other hand, are convex and 

 more or less cylindrical insects with a large heavy pronotum, 

 the anterior border of which is often slightly reflexed and 

 bounded laterally by the upwardly projecting lateral wings. 



Mr. Distant, in his ' Insecta Transvaaliensia,' has recently 

 published an interesting field-observation on tlie habits of a 

 species identified as Pilema thoraci'ca, Walk. A female 

 accompanied by several larvse was taken from the bottom of 

 a neat round hole in the ground about 6 inches in depth ; 

 there were half a dozen such holes in about half an acre, and 

 all contained families of this species. I have no doubt but 

 that all the species of this genus have adopted this mode of 

 life, and that the pronotum is the part of the body that is 

 used in excavating the burrows, for on examining some 

 specimens of P. refiexa, Walk., and P. hebetata, Sss. & Z., 

 in the British Museum, I found that in these the channel 

 between the pronotal disk and lateral bands was simply 

 choked with earth. It is not unreasonable to assume that 

 the heavy shovel-like pronotum of Pilema has been evolved 

 in response to a change of habitat. Turning now to the 

 other end of the series of cockroaches considered, we find 

 that the species of Derocalymma are the most flattened 

 members of the whole family, and in correspondence with 

 this depressed form it is no surprise to learn that they live 

 under heavy stones. The advantage of the flattened form, 

 enabling the insects to slip through narrow crevices and to 

 lie in security in a circumscribed shelter beneath a stone too 

 heavy for any but a relatively powerful enemy to move, is 

 obvious ; and, again, it is not unreasonable to assume that 

 the highly modified pronotum of Derocalymma is a result of 

 a change of habitat. There is no information forthcoming as 

 to the habits of Bantua and Cyrtotria-, some species of the 



