NO. 8 INSECT ABDOMEN SNODGRASS 3 



as a whole recognizes dorsiventrality as a primary factor in structural 

 differentiation, and accepts the series of limb bases along the sides 

 of the body as marking the anatomical distinction between dorsum and 

 venter. The plan, in the abstract, can scarcely be questioned as mor- 

 phologically sound. It is only in its practical application that questions 

 of interpretation come up, and if, as applied by the writer, the scheme 

 here and there conflicts with current local interpretations based on 

 former opinions, justification for the interpretive alterations proposed 

 must be found in the degree of improvement given to the general or 

 perspective view of insect structure as a whole. The application of 

 the plan, as carried out in this series of papers, will at least open 

 a way by which the teacher in entomology may conduct his students 

 at once straight through the insect without giving the impression that 

 the head, the thorax, and the abdomen are each a region foreign to 

 the others ; if followed in descriptive entomology it would furnish a 

 basis for a common nomenclature. The plan gives at once a unified 

 and a simplified concept of insect morphology, and many facts un- 

 questionably are in its favor ; but there is no pretense here made to 

 decree that order and simplicity in any particular pattern are synony- 

 mous with truth. 



I. GENERAL STRUCTURE OF THE ABDOMEN 



In the study of the abdomen it is highly important to be able to 

 identify the several morphologically distinct surface regions. These 

 regions are the dorsum (fig. i A, B, D) lying above the region of 

 the primitive limb bases; the venter (F) lying below the limb bases, 

 and the podial, or pleural, regions (P) which are the areas of the 

 limb bases themselves (LB). The dorsum is separated from the 

 pleural region by the dorso-pleural line (a-a) ; the pleural region is 

 separated from the venter by the pleuro-ventral line (b-b). Much 

 confusion exists in descriptive works on entomology because the 

 positions of these lines have not been clearly perceived, and, as a con- 

 sequence, names have been applied to certain parts that do not properly 

 belong to them. By observing whatever landmarks are present, how- 

 ever, it is usually possible to determine the limb base areas on the 

 abdominal segments ; the rest of the surface is then apportioned to 

 the dorsum and to the venter. 



The dorso-pleural line (fig. i B, a-a), beginning on the head (fig. 

 2 A), separates the lower edge of the cranium from the bases of 

 the gnathopods (mandibles and maxillae) ; on the thorax (figs, i C, 

 2B), it goes dorsal to the subcoxal plates of the leg bases, dipping 

 down between them where necessary to pass beneath the spiracles ; 



