NO. 6 INSECT ABDOMEN — SNODGRASS 3 1 



GENERAL PLAN OF THE ABDOMINAL MUSCULATURE 



A review of the literature cited above gives a fairly comprehensive 

 survey of the abdominal musculature of insects in most of the principal 

 orders. There are notable blanks, however, since such important 

 orders as Neuroptera and Trichoptera are omitted entirely, and 

 adult Lepidoptera and Diptera have been given scant attention. On 

 the other hand it is gratifying to find that we have, as a basis for 

 a comparative study of insect myology, very full accounts of the body 

 musculature of the Odonata, Ephemerida, and Orthoptera. In the 

 Apterygota, we are indebted to Berlese for an excellent study of the 

 muscles in Protura, to Lubbock for a description of the collembolan 

 musculature, and to Grassi for brief descriptions of the characteristic 

 differences in the nuisculature of representative genera of Dicellura 

 and Thysanura, to which is added in this paper an account of the 

 abdominal muscles of Heterojapyx ; but a more complete study of the 

 musculature of Machilidae and Lepismatidae, and perhaps of Cam- 

 podea, is still to be desired. When we look to the papers treating 

 of holometabolous larvae, we find again satisfactory and in some cases 

 complete accounts of the body musculature in Coleoptera, Lepidoptera, 

 Hymenoptera, and Diptera, but note with regret a lack of information 

 on Neuroptera and Trichoptera. 



To present here even a summary of the details known concern- 

 ing the abdominal muscles of insects would occupy an unwarranted 

 amount of space. A careful review of the facts to be obtained from 

 the works above listed, however, shows that we may with confidence 

 make certain broad generalizations concerning the fundamental plan 

 of the abdominal musculature of adult pterygote insects. The basic 

 plan is found to be simple ; but, as so often occurs in insect mor- 

 phology, more difficulties are encountered in finding suitable terms to 

 express the facts than in discovering the facts themselves. 



Voss (1905) classified the abdominal muscles as longitudinal mus- 

 cles, transverse muscles, and lateral muscles (Flankenmuskeln). This 

 classification is logical inasmuch as it probably conforms with the 

 primitive arrangement of the fibers. The muscles of the so-called 

 longitudinal groups, however, do not always preserve a lengthwise 

 arrangement; they are often strongly obliciue, and some of them 

 frequently take a transverse position. The lateral muscles are desig- 

 nated " dorsoventral " muscles by many writers, but, though their 

 attachments are usually dorsal and ventral, some of their fibers com- 

 monly run in an oblicjue direction. The lateral muscles have also 

 been termed "transverse" muscles, but, as Samtleben (1929) points 



