1909.] NATURAL SCIENCES OF PHILADELPHIA. 43 



In most Hymenoptera, the prothoracic pleura are very closely con- 

 nected with the cervical sclerites, and in many insects of this family 

 the pleura of the pro thorax are almost indistinguishably fused with 

 the lateral cervicals. On this account the term cervico-propleura 

 has been here applied to the lateral portions of this region in the 

 Hymenoptera. 



If one observe a Macroxyela (or a Tenthredo), a wasp and an ant, 

 in the order above mentioned, it may readily be seen that the cervico- 

 propleura (fig. 13, C.Pl) gradually approach one another on the ventral 

 surface, and almost completely conceal the small presternum (fig. 14, 

 S'). In such a case, the ventral portion of the cervico-propleura is 

 almost without exception incorrectly termed the " presternum, " and 

 even so careful an observer as Janet, '98, has been deceived in this 

 respect. Indeed, Rheinhard, '65, terms the entire cervico-propleuron 

 the presternum, as does Schmiedeknecht, '07, and a number of 

 others. 



As has been stated, the cervical sclerites may represent the "inter- 

 segmentalia, " and, in addition, certain of the neck plates doubtless 

 owe their origin to mechanical friction. This brings us to the question 

 of the formation of sclerites in general. The prevalent opinion is that 

 the segments originally consisted of chitinized rings, which became 

 split between the pleura and tergal region to accommodate the wing; 

 and the pleura became separated from the sternal region to make room 

 for the leg. This view, however, is not borne out by the facts of onto- 

 genetic development, nor by the results. of comparative morphological 

 study. In all of the very active larvae studied, the sclerites of the ster- 

 num and pleura form as islands (pi. IV) which later unite to form the 

 chitinous integument. Again, in the adult stages of the lowest insects, 

 such as Japyx for example, even though there are no wings present, the 

 segments are not solid rings interrupted only at the base of the leg; 

 but consist for the most part of small individual sclerites. It is like- 

 wise the case that within the same family the sclerites of the general- 

 ized forms are more numerous and distinct, while in the more specialized 

 forms there is a marked tendency toward a fusion of the sclerites to 

 form a solid ring; and even the segments themselves tend to become 

 closely united. 



With regard to the origin of the sclerites, there are a number of fac- 

 tors which might give rise to such chitinous plates. Among the chief 

 of these causes is doubtless the stimulus of muscular tension, which 

 would serve to produce a chitinized thickening of the integument at 

 the points of origin and insertion. The sclerites thus formed would not 



