118 BRITISH LEPIDOPTERA. 



in the pupal condition, the males measured only 35mm.-37mm., 

 and the females 36mm.-39mm., no hybernation of the larva 

 having taken place. On the other hand, eggs of Dendrolimus 

 pini, $ 59mm. and $ 74mm., yielded descendants expanding 

 $ s 65mm.-68mm., $s 84mm.-86m.rn., after 150-172 days of larval 

 feeding, and 25-37 days in the pupal condition. Standfuss 

 further notes {doc. cit.) that the form and appearance of the 

 insects, produced by means of forcing under a raised temperature, 

 were most marked in Eutricha populifolia of all those species upon 

 which experiments were made. The second and third broods of this 

 species, viz., var. obscura, Heu. (aestiva, Stand.), and var. autumnalis, 

 Jaenich, exhibit, especially the latter, a more deeply-indented border to 

 the wings than does the form resulting from hybernated larvae. 

 . . . . Both these varieties, however, reared in shorter time 

 and also at a higher temperature, become darker than the ordinary 

 torm of E. populifolia (obtained from hybernated larvae) by an increase 

 in the dark elements of the pattern. He further suggests that 

 Gastropacha (Epicnaptera) tremulifolia and Eutricha populifolia, which, he 

 says, resemble each other extraordinarily in the imaginal, and to 

 a certain extent in the earlier, stages, have possibly been 

 differentiated as species as a result of the fact that, in earlier 

 periods of the earth's history, their ancestors, whilst in the larval 

 condition, did not react in the same manner to changes of 

 temperature, and that the difference exhibited by G. tremulifolia, 

 hybernating as a pupa, and E. populifolia, hybernating as a larva 

 (and rapidly emerging from' the pupa), may have arisen as a necessary 

 consequence of a divergence in the larval habit at the time of 

 the establishment of the species. 



The sexual dimorphism, so distinctly marked in the Macrothy- 

 laciids and Pachygastriids, particularly the specialised features 

 exhibited in the antennal structure, and the difference in colora- 

 tion (being, as one would surmise, due to a response to the 

 different habits exhibited by the sexes, the males flying swiftly by 

 day, and assembling to the females in the afternoon or at dusk), is 

 largely lost in, or has never been gained by, many of the 

 higher Eutrichids — although retained to a certain extent in 

 Cosmotriche — the difference in the habits of the sexes being less 

 marked, and a much greater resemblance between the sexes being 

 noticeable in the general shape and coloration of the wings. The 

 males of C. potatoria, however, differ largely from the females, but, in 

 Dendrolimus, Odonestis, Gastropacha, and Eutricha, the coloration 

 and shape of the wings approach somewhat in the sexes, evidently 

 in response to their resorting to a common habit for the purpose 

 of protection, the resemblance to dead leaves being undoubted. The 

 sexual difference in size is, however, maintained, for the females have a 

 great body, necessary to hold a considerable number of com- 

 paratively large eggs, and they have also a proportionally large 

 wing-area to carry the body. In some Eutrichids (e.g., the Pinarids) 

 the males are exceedingly small compared with the females of the 

 same species. As a result [except in Cosmotriche potatoria, which, 

 especially in its strongly developed antennae, wide variation in ground 

 colour (yellow to deep brown), a tendency by polymorphic colour- 

 variation for the extremes of colour normal to either of the two sexes 



