INSECT SPINASTERNAL MUSCULATURE — CIIADWICK I35 



reported by Mayer (1954) in four genera of Mallophaga, as muscle 

 No. // trm 2, which she describes as cx^rcx-y ; i.e., the former spina- 

 sternal attachment has been lost. The promotors are not known to 

 occur in this form in any other group of insects. 



One naturally wonders whether there exist prothoracic homologues 

 of the spinacoxal promotors of the second and third legs. Maki 

 (1938) may have found them in Collembola (Ncannra, No. 5<?) and 

 Diplura {Lcpidocampa, No. jd). Other insects apparently have no 

 structure that stands in place of a cervical spina, so that searching 

 elsewhere for strictly homologous muscles would seem fruitless. 

 Nevertheless, Badonnel (1934) considered the cruciate cervical pro- 

 motor, icv-cxrX in my notation, of Stenopsocus (No. Xi) serially 

 homologous with the spinal promotors that he found in the ptcro- 

 thorax. His conclusion was based partly on the similar innervation. 

 Barlet (1954) has regarded the cruciate coxotentorial muscles (Nos. 

 P9, 100) of Lepisma as serial homologues of the spinacoxal promotors 

 of the mesothorax and metathorax (Nos. 106, 107; 112, 11^; 

 respectively). 



Now, cruciate cervicocoxal muscles, undoubtedly homologous with 

 those of Stenopsocus and Lepisma, have been described from nine 

 other orders, all among the Pterygota. These muscles may surely be 

 looked upon as functional replacements for the missing prothoracic 

 spinal promotors, but one doubts that they can be wholly homologous 

 with them in the morphological sense. The resolution of this question 

 should hang together, it seems, with one's interpretation of the possible 

 homology between spinal and cruciate remotors (see section 11 

 above) ; and here the evidence, such as it is, does not favor the pro- 

 posed homology. A major stumbling block is the fact that both types 

 of remotors are sometimes found in the identical segment. Although 

 this is not yet the case for the promotors, one still cannot accept 

 Badonnel's argument from the innervation, which is simply of the 

 type characteristic of most intersegmental muscles and hence not de- 

 cisive as to the point at issue. Nor can one agree with Barlet that the 

 crossing point of the right and left coxotentorial muscles represents 

 the missing cervical spina, for there are too many instances in other 

 intersegments where cruciate muscles and authentic spinae exist to- 

 gether. Barlet's further suggestion, that the "long" coxotentorial 

 muscles have been formed by coalescence of former spinacoxal muscles 

 with some other spinal element, deserves consideration, but it again 

 meets the old difficulty of explaining the coexistence of spinal and 

 cruciate remotors in the same segment. Once again it seems that only 



