Fig. 4.—Generalised sketch of Cockroach wing-cover.
About eight hundred species of Cockroaches have been defined, and to facilitate their arrangement, three groups have been proposed, under which the different genera are ranked.[13]
Group 1. Both sexes wingless (Polyzosteria).
Group 2. Males winged, females wingless (Perisphæria, Heterogamia).
Group 3. Both sexes with more or less developed wings (about 7 genera).
In Group 3 occur the only two genera which we shall find it necessary to describe—viz., Blatta, which includes the European Cockroaches, and Periplaneta, to which belong the Cockroaches of tropical Asia and America.
Genus Blatta. A pulvillus between the claws of the feet. The seventh sternum of the abdomen entire in both sexes; sub-anal styles rudimentary in the male.
Genus Periplaneta. Readily distinguished from Blatta by the divided seventh abdominal sternum of the female, and the sub-anal styles of the male.