In this article, attention will be drawn to some of these features as seen in the ventral intersegmental musculature of the thorax. Although this system of muscles is far from homogeneous morphologically, it provides a convenient segment of the total thoracic musculature for analysis. Other remains of the third spine and its musculature have been found, but not recognized as such by several students, in a number of other insects.
Each of the 19 species examined here has all the muscles listed in the table, except as noted in the comments below or in the text. In listing two fureal posterior rotators of the third coxa, Maki (1938) suggests that one of them may be serially homologous with the spinal posterior rotators of the other legs. However, the transverse muscles are represented in the abdomen by the muscle attachments of the ventral diaphragm on the anterior lateral corners of the second abdominal sternum.
The lateral suspensions of the ventral diaphragm are not clearly visible in the abdominal intermediate segments behind the first; and in general the ventral diaphragm of cockroaches is much less extensive than that of some other insects, for example, phasmids and the sharp Orthoptera. Spinal promoters of the mesocoxa and metacoxa (isps-cxs, 2sps-CX3) are also common in cockroaches, as in other primitive forms. Both spinal promoters are also absent in Cryptocercus, which shares to some extent the tendency of these genera for hypertrophy of the transverse muscles of the first spina, although it clearly differs from them in certain other respects.
The spinaspinal muscles are the only part of the ventral longitudinal musculature that has obviously preserved its primary relationships in the blattida.
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THE INTERSEGMENTAL LATEROSTERNITES
The principal primary muscular relations of the US are tolerably well preserved in the typical abdominal intersegment, provided that a secondary extent of sclerotization has here indistinctly united the intersegment with the following sternal segmental plate, from which the former intersegment now forms antecorasssta ( , 1929). The U's are represented here at the anterolateral angles of the terminal abdominal sterna, which in many insects exhibit the muscular relationships described above (Ford, 1923; Maki, 1938). In the intersegments following each of the three thoracic segments, the situation is rarely so transparent.
One gets the impression that, even in the most primitive forms that have come into question, the muscles of the US have already been subjected to extensive shifting and reduction, while in more recent insects only a few scattered remains represent the original role of the Us as attachment sites for part of the longitudinal body muscles. In addition, we find some muscles that originate on the thoracic Us or on their current equivalents, whose insertions are segmenta and which have no counterparts in the boneless abdomen. Here one has to be content for the present with the assurance that the former OEs usually are in one way or another.
These problems are well illustrated in cockroaches, in which the musculature of Us, although rich compared to that of. Cockroach abdominal muscles that appear to belong to the Us complex include (a) transverse muscles; (b) coxalandfurcal cruciate muscles; (c) some other furcal muscles; and (d) abdominal spinasternal muscles. Transverse Muscles.— The transverse muscles of the thorax usually have a median attachment to the back, and are therefore discussed in section i,a, above.
As already noted, the abdominal relationships support the view that the lateral attachments of the transverse muscles are morphologically intersegmental, i.e. based on comparative evidence from other arthropods and the scarce em-.
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THE FURCAE
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The complex endosternal structures of the Apterygota provide a number of examples, because, as described by the Belgian authors cited above, many ele- endosternal. In the opinion of the present writer, still other parts of the endosternum which are invariably ligamentous in the apterygote species hitherto studied are homologous with muscles, such as the transverse muscles, which persist as contractile elements in some primitive Pterygota and as ligaments in others. . . Another clear set of examples of muscle replacement by endoskeletal structures is found in the later history of the sternal wings themselves, for example in the development of furcopleural fusion, which occurred independently in multiple lineages.
These muscles, deprived of their original efficiency as contractile organs, are doomed to disappear, unless they happen to retain some value in the role of static supports or struts. Returning to the narrower problem of the nature of the pectoral arms and their longitudinal musculature, we can emphasize that the arms in Apterygota are represented by ligamentous bands connecting the thoracic endosternum, which is mainly intersegmental in character, with the corresponding anterior segmental sternal regions (references in Barlet, 1954 ).
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NO. II THORACIC MUSCLES OF COCKROACHES — CHADWICK 21
GENERAL DISCUSSION
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SUMMARY
In general, there are very minor differences between species with regard to the presence or absence of individual muscles in the category examined, although there are numerous differences, some striking, in the relative size and proportions of the various muscles. The current ventral intersegmental thoracic muscles had their primary attachments on the spinae, on the intersegmental latero-sternites, or on the precursors of the furcal apophyses. Remnants of the intersegmental laterosternite musculature are present, but some of these muscles now have segmental attachments, and others are represented by noncontractile ligaments.
Attention is drawn to the postcoxal ligaments running between the furcal apophysis and the immediately following intersegmental laterosternite in each of the thoracic intersegments, and to the significance of these former muscles in the development of the postcoxal bridges of higher insects. It is suggested that one attachment of these muscles was on the segmental sternum, the other on a common intersegmental junction of several other muscles, including elements of the longitudinal ventral group. The blattid prothorax is an example of a stage in the evolution of the typical furcal apophysis, when the postulated pectoralis muscle was still in a liga.
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ACKNOWLEDGMENT
GLOSSARY OF ABBREVIATIONS
NO, II THORACIC MUSCLES OF COCKROACHES — CHADWICK 2$
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EXPLANATION OF FIGURES
NUMBERING OF MUSCLES IN FIGURES I-18 Number Muscle
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