1891.] 177 



the GeometrcB, and in some o£ the Diurni, I have not found it in any of 

 the other great Macro groups, and have only met with it in the genua 

 Elachista among the Micros. The first important modification, and 

 one that I think includes the greater proportion of the female insect 

 in, perhaps, all the great groups, consists in the fusion of the two 

 separate plates into a single strong piece or ring, not always quite 

 perfect, owing to the occasional presence of a narrow gap on the 

 ventral face occupied by membrane alone. The ring is still wholly 

 external, and is clothed and frilled with scales. A further advance 

 follows from retraction of the ring, more or less under the 7th seg- 

 ment, and by the loss of all the scales but the frill. Nevertheless, 

 the presence of the frill indicates that it has not yet quite lost its 

 external character, but continues, as in the foregoing forms, to be part 

 of the outer wall. Now% by retracting the part completely under the 

 7th segment, and by removing the last remnant of the scales, the 

 change is completed, and it is transformed from an external segment 

 to an internal one, or sheath, and becomes an appendage to the 

 ovipositor ; whilst the oiSce of forming the abdominal outlet is trans- 

 ferred to the 7th segment. At the same time with its conversion into 

 a sheath, it usually becomes longer in proportion to its width, and 

 acquires the shape of a tube rather than that of a ring. As I shall 

 have an opportunity later on, when describing the parts in individual 

 insects, of dealing with some of the extreme forms of the sheath, I 

 will content m}self for the present with merely observing that it 

 would be scarcely possible, without some such survey as the above, to 

 recognise its true homologies, and that in all cases the sheath is, beyond 

 dispute, the metamorphosed 8th segment. 



Let us now briefly look at the homologies of the ovipositor. This I 

 organ consists of t'vo distiiu-t i)arts : one, inner and visceral, and the I ' 



other, outer and skeletal. The visceral portion is a fleshy hyaline o'^ 

 duct, a prolongation from the cloaca, and serves for the passage of the ' 

 excrementa as well as for the delivery of the egg. The outer or 

 skeletal part is the portion that has most importance for us. In the 

 great majority of cases it may be described as a pair of chitiuous 

 plates, which embrace and take hold of the visceral tube, and are the 

 means by which its protusion and retraction are effected. It must 

 not, however, be concluied that the plates, though separate, are dis- 

 connected, for they are united to each other by membrane, just as are 

 their homologues, the abdominal ])lates, so that, were the visceral tube 

 removed, an outer or skeletal one would still remain. Moreover, ala 

 the plates themselves are mn-ely chilinous deposits in this membrane, 



