208 THE COCKROACH 



One principal difference here alluded to the exact resem- 

 blance, except in minor details, of the front and hind wings, 

 and, as consequent therewith, equal diaphaneity in both is found 

 indeed in all palaeozoic insects, with exceedingly few exceptions;* 

 it is one of their most characteristic and pervading peculiarities. 

 It marks one phase of the movement in all life from homo- 

 geneity to heterogeneity from the uniform to the diverse. In 

 the Cockroaches of to-day a few are found in which the tegmina 

 are nearly as diaphanous as the hind wings ; but in the great 

 mass the texture of the tegmina, as in Orthoptera generally 

 (excepting most Gryllides), is decidedly coriaceous ; and in 

 some, e.g., Phoraspis, the veins are nearly obliterated in the 

 thickness and opacity of the membrane, so as to resemble many 

 Coleopterous elytra. 



Three principal differences have been noticed between the 

 ancient and modern forms of Cockroaches. Doubtless others 

 could be found were we able to compare the structure of all 

 parts of the body ; and perhaps future research and more happy 

 discovery may yet bring them to light ; at present, however, 

 we are compelled to restrict our comparisons to the wings 

 alone. 



First, we have to remark the similarity of the front and hind 

 wings in the ancient types : a similarity which extends to their 

 general form (the extended anal area of the hind wings in 

 modern types being as yet only slightly differentiated) ; their 

 nearly equal size (a corollary, to a certain extent, of the last) ; 

 the general course of their neuration (true, in a limited sense 

 only, of modern types) ; and the complete transparency of the 

 front as well as of the hind wing. 



Second, the same number of principal veins is developed in 

 the front and hind wings of ancient Cockroaches ; while in the 

 front wings of modern types two or more of the veins are 

 blended, so as to reduce the number of the principal stems 

 below the normal, the hind wing at the same time retaining its 

 original simplicity. These principal veins are six, counting the 

 marginal vein, which here merely thickens the anterior border, 

 as one ; to use the terminology of Heer, and starting from the 

 anterior margin, they are the marginal, mediastinal, scapular, 



* A few elytra of Coleoptera are recently announced from the Silesian "culm." 



