NO. I INSECT THORAX — SNODGRASS II 



frequent in higher insects, the compound, forked apodeme has been 

 named the furca, but the presence of two independent processes un- 

 doubtedly represents the more primitive condition. In some cases, the 

 sternal arms, or apophyses (figs. 3 B, 4, SA), arise from a transverse 

 ridge of the sternum, as in the prothorax of Acrididre, and the exter- 

 nal groove of the ridge then divides the sternum into two parts ; but 

 again, a division of the sternum may occur anterior to the bases of 

 the furcal arms. The two parts of the sternum have been called some- 

 what loosely the sternum and sternellum, but since the first name 

 should be reserved for the entire sternal plate (fig. 3 A, 6"), the terms 

 bosisfcrnuiii and furcisfemuin (Bs and Fs) proposed by Crampton 

 ( 1909) are to be preferred, though the first sternal sclerite is not 

 *' basal " and the furca is not necessarily a process of the second. 



In addition to the sternal arms or furca, there is often present in 

 the lower orders of winged insects a median sternal process known as 

 the spina (fig. 3 B, Spn) situated behmd the paired apodemes. Some- 

 times the spina appears to be carried by the posterior margin of tlie 

 furcisternum, but typically it is borne on an independent sclerite, the 

 spinisternum in Crampton's nomenclature. Another sclerite, the post- 

 fur cist ernum, is found in rare cases between the furcisternum and the 

 spinisternum, or extends as a fold laterally, where it may bear a pair 

 of small processes known as the ftircillce. It is probable, however, 

 that both furcisternum and spinisternum are parts of one poststernal 

 region. 



In general, then, we may say that the ventral chitinization of a 

 thoracic segment consists of a principal plate, the true sternum (fig. 3 

 A, S), and of one or two poststernal sclerites constituting a post- 

 sternimi (PS), the latter visually associated with, or incorporated 

 into, the posterior part of the sternum proper. There is reason to 

 believe, as will be shown later (page 21), that the poststernal parts 

 are primar}- intersegmental elements that once constituted the pre- 

 costa and antecosta of the sternum following. At any rate, this 

 assumption explains the apparent reversal in the attachments of the 

 ventral thoracic muscles, which, as the poststernal sclerites are lost, 

 1)ecome transferred to the posterior parts of the preceding sterna and 

 finally to the furcisternal apodemes. 



The two divisions or regions of the sternum proper usually differ 

 considerably in shape and size. The basistemum lies mostly before 

 the bases of the legs (fig. 7 B, Bs2, Bso,) and may become expanded 

 laterally or fused with a precoxal part of the pleuron (fig. 3 A, Acx). 

 Its anterior part is sometimes differentiated as a presternum, being 

 separated from the rest by a suture, and an internal ridge simulating 



