100 HOMOLOGIES OF CERTAIN 



suggest what appears to be a parallelism of type in this respect 

 between the Crustacea and the Inseda. I have mounted a speci- 

 men of the common wood-louse, together with the cockroach, on 

 one slide, and have given drawings of the same. See lower 

 portion of Plate V., given in the January part of this Journal. 



In the wood-louse will be observed a succession of dorsal 

 plates projecting laterally beyond the body of the creature, the 

 projecting portions being darkly shaded in the drawing. So 

 also in the larval cockroach, by its side. The three thoracic 

 segments are each covered by a continuous dorsal plate, in which 

 as yet no indication of separation of parts is seen. These like- 

 wise project laterally beyond the body, the projecting portions 

 being likewise darkly shaded. 



Now, in the wood-louse, it is perfectly manifest that the whole 

 breadth of each segment is the homologue of the whole breadth 

 of every other. It would be absurd to regard the whole breadth 

 of one as homologous with, say, the central grey portion only of 

 the succeeding, and yet this very thing (*) appears to be done in the 

 case of insects generally, and that of the cockroach in particular. 

 In Figs. 3 and 4 on the plate will be found drawings of the 

 dorsal surfaces of the thoracic segments of this insect in more 

 advanced stages. In the first the dorsal surfaces are still continu- 

 ous, but the markings now visible upon them distinctly foreshadow 

 the approaching separation of the wings. In the male this sepa- 

 ration is complete, but in the female the tegmina alone are com- 

 pletely separated, the wings proper being still only indicated by 

 the markings referred to. The fourth figure shows the dorsal 

 surfaces of the female thoracic segments in this condition. 

 Now, I have always been taught to regard the great shield-like 

 plates of the pro-thorax of this and other orthopterous and coleop- 

 terous insects as constituting in its entirety the dorsal plate proper 

 of the segment, while at the same time I have been equally led to 

 regard the dorsal plate of the next or meso-thoracic segment as 

 that portion only of the upper surface which lies between the 

 wings. Thus it comes to pass that the dorsal plate of the pro- 

 thorax occupies its whole breadth, both the grey and red portions, 

 in my drawing (Fig. 4, PI. V.), while the dorsal plate of the next 

 fills only its central grey portion, and this brings me to my state- 



