1916] Thoracic and Cervical Sclerites of Insects 71 



The furcasternite is situated directly behind the vera- 

 sternite and is usually a much smaller sclerite, although quite 

 variable in shape in different insects (Fig. 7, fs). It is usually 

 fused with the verasternite, but may be detached as in the 

 prothorax of Periplaneta (Fig. 3, fsi). In insects where fusion 

 of the furca- and verasternite occurs, the former is often divided 

 longitudinally by the median groove, exactly as in the vera- 

 sternite (Fig. 14, fs). This median infolding of the integument 

 does not occur in primitive insects and is probably a secondary 

 modification of the more specialized forms. 



Internally the furcasternite bears two processes or furca. 

 These are usually hollow, strongly chitinized apophyses and are 

 always present (Fig. 3, f). The furcal arms usually abut against 

 the apodemes (pleural arms) and very frequently are closely 

 united with them, thus forming a strong internal connection 

 between the pleuron and sternum. 



Externally the furcasternite is easily distinguished, even 

 when fused with the verasternite. Sometimes in such insects 

 as Capnia, it is connected with the epimeron (Fig. 4, fsi). This 

 is brought about by a narrow sclerite (postepimeron), which 

 extends from the epimeron behind the coxa and joins the 

 furcasternite, thus forming a connecting bridge. When the 

 precoxale and verasternite are also fused, a complete ring is 

 formed inclosing the coxa. 



The pleural suture, together with the suture which extends 

 between the veracoxa and meron and the one which separates 

 the furca and verasternite, present a condition which has 

 given rise to several double segment theories, but it does not 

 necessarily follow that these sutures mark a division between 

 two segments, as previously discussed. 



The furcasternite in the prothorax of Periplaneta is a 

 single, transverse, oval sclerite (Fig. 3, fsi). In the meso- and 

 metathorax it is Y shaped and fused with the verasternite 

 (Fig. 3, fs2, fsa). In some forms it is also fused with the spinas- 

 ternite (discussed later) as in some Lepidoptera, etc., while in 

 others, such as in the prothorax of Leuctra, it is divided into 

 two sclerites. Thus there are a few modifications in different 

 insects. 



There are indications of a fourth division of the sternum 

 in a few insects, but it is usually absent. In the prothorax of 

 Capnia, it is a single, large, oval sclerite, lying behind the 



