INSECT SPINASTERNAL MUSCULATURE^CIIADWICK I3I 



segment to an apodeme at the base of the first abdominal appendage. 

 If the muscle in question is truly homologous with isps-fuo, 2sps-fus, 

 as seems likely, the apodeme of attachment is to that extent homolo- 

 gous with the sternal arms of the thoracic segments. 



6 and 7. The muscles listed under these numbers are of such infre- 

 quent occurrence that one is persuaded to look elsewhere for their 

 derivation, as will be explained presently ; yet 2sps-Iils is character- 

 istic of cockroaches, where it was described as 2sps-SiA by Chadwick 

 (1957, No. 21). He accepted its equivalence to No. iiib of Walker 

 ( 1938) in Grylloblatta, for reasons that have been given in section 4 

 above. Elsewhere, the muscle has been noted only in adult Corydalus; 

 it is not present as such in the larva of this species (Chadwick, 

 unpublished). 



The same author (1957) has outlined how, in certain cockroaches, 

 2sps-Iils may give rise to 2sps-IIils by incorporating an additional seg- 

 ment of the abdominal longitudinal ventral musculature. Though the 

 resulting muscle has not been found in any other insect, so that it 

 seems peculiar to cockroaches, the here evident mode of its formation 

 suggests a possible origin for other long muscles, such as its parent, 

 2sps-Iils. The latter could easily have arisen through coalescence of 

 2sps-sils with pls-Iils, both of which muscles are of types serially 

 repeated through several segments in primitive insects ( for 2sps-^Us, 

 see section 4 above). The facts observed in Corydalus make it certain 

 that this process is what takes place. The immature form of this in- 

 sect has muscles 2sps-lig^, the equivalent of the more usual 2sps-sils, 

 and ligs-Iils (for jils-Iils), but lacks 2sps-Iils, which comes into exist- 

 ence, as mentioned above, only in the adult. Now, the larval muscles 

 suspended on Ug^, which is in this insect the partial equivalent of S^P^- 

 Sils, have already dissolved their formerly direct attachments to the 

 integument and are therefore, so to speak, floating in the body cavity. 

 During metamorphosis, the ligament finally disappears, and what was 

 two muscles joined end to end becomes a single element. 



We believe then that 2sps-Iils has been formed from 2sps-sils by 

 simple addition of one segment of body musculature, just as 2sps-IIils 

 in cockroaches was derived from 2sps-Iils, as shown previously. We 

 have outlined the process in some detail, not only because of its in- 

 trinsic interest, but also because it seems likely that it is a model for 

 the derivation of certain other spinasternal muscles, some of which 

 are to be discussed in section 10 below. 



8. A muscle from the second spina to the first furca has been de- 

 scribed from 12 orders (table i) and may therefore be regarded as a 



