INSECT SPINASTERNAL MUSCULATURE — CHAD WICK 1 39 



7. Spinaspinal muscles are only rarely preserved; but, as part of 

 the original longitudinal body musculature, their presence must never- 

 theless be regarded as a primitive sign. 



Although the spinasternal musculature thus gives evidence of a 

 fundamental likeness among the thoracic segments, we must not over- 

 look the fact that the three dififer at present in many important re- 

 spects. Their differentiation from one another began very early, 

 under stress of the somewhat different forces to which each was sub- 

 jected; and even the spinasternal musculature shows unmistakable 

 traces of such effects. Our reconstruction of the basic spinasternal 

 pattern affords but a glimpse into the state of aflfairs that prevailed 

 before the functions and hence the structure of the present thoracic 

 segments began to diverge ; from this viewpoint our list may even be 

 thought of as preinsectan in its nature. Certainly alterations specifi- 

 cally affecting the cervical region must have occurred in connection 

 with the development of the characteristic architecture of the head, and 

 these must therefore long antedate the emergence of the insects as a 

 class. Changes in the anterior abdominal region must also have taken 

 place very early, as an accompaniment of restriction of the locomotor 

 function to the thorax. 



It is the subsequent history of the spinasternal musculature that 

 should help to illuminate the interrelationships of modern insects ; and 

 in Part II of this paper I have used the data here presented for such 

 an analysis. 



PART II. THE RELATIONSHIPS OF SOME ORDERS OF 

 INSECTS, AS INDICATED IN THEIR SPINA- 

 STERNAL MUSCULATURE 



Review of existing information on the spinasternal muscles of the 

 Thysanura and 24 pterygote orders prompts the question, to what 

 extent might this and similar knowledge be used to illuminate the 

 interrelationships of the insects concerned? 



The pertinent facts about these muscles have been given in detail 

 in Part I of this paper, to which the reader is referred also for ref- 

 erences to the original sources. As concluded in that study, all present- 

 day orders are traceable ultimately to a single stock of insectan or 

 preinsectan forms whose spinasternal musculature included not less 

 than the 28 elements listed below in table 2. Of these elements, we find 

 27 represented in the Thysanura, 19 each in the Blattariae and Meg- 

 aloptera, 16 each in the Orthoptera and Coleoptera (larval dytiscids) ; 

 and so on, down to i in Hemiptera and o in brachycerous Diptera. 



