NO. 6 INSECT ABDOMEN SNODGRASS 



29 



may assume that the primitive somatic fillers were all 'nitra-scguicntal 

 in arrangement, as they are in the Annelida. With the acquisition 

 of secondary segmentation in arthropods, however, consequent upon 

 the development of sclerotic plates in the body wall, the princq:)al 

 longitudinal fibers l)ecame functionally intersegmental (fig. 2 F). The 

 body of the animal can thus be shortened by a telescoping of its seg- 

 ments Ijrought about by contraction of the longitudinal muscles, and 

 it can be compressed by contraction of the lateral dorsoventral muscles. 

 In most cases the opjwsite movements result either from the elasticity 

 of the body wall, or from ]M-essure generated by contraction in one 

 part of the body transmitted to another through the medium of the 

 body liquid and the visceral organs ; but in many insects a dilator ap- 

 paratus is developed in which certain muscles in both the longitudinal 

 and dorsoventral systems become antagonistic to the retractors and 

 compressors. 



The abdominal musculature of adult insects is simpler than the 

 thoracic musculature because of the absence of leg muscles. There is 

 no evidence that the definitive lateral muscles of the abdomen have 

 been derived from the body muscles of the limbs. Muscles of the 

 movaljle parts of the abdominal appendages, as will l)e shown in the 

 next section, arise generally within areas of the I)ody wall that may 

 he attributed to the limb bases (figs. 32 V>, C, 34 B, 36 D), except 

 the muscles of eversil)le or retractile sacs which in some cases have 

 evidently extended to the dorsum. The general segmental ])lan of 

 the abdominal musculature is usually repeated with only minor varia- 

 tions in each of the visceral segments ; in the genital and postgenital 

 segments it is more or less obscured by special modifications. 



A rather simple scheme of abdominal muscle arrangement prevails 

 throughout all adult pterygote insects ; but in the Apterygota and in 

 larval forms of holometabolous insects the musculature may lie hi«hlv 

 complex. Some students regard the complex types of musculature as 

 representative of a primitive condition from which the simpler types 

 have been derived by elimination. There are reasons, however, for 

 taking the opposite view, as will later be sliown. 



Something is known of the abdominal musculature in most of the 

 princijml orders of insects ; but the Odonata, Orthoptera, Coleoptera, 

 Hymenoptera, and the larvae of I.epidoptera and Diptera have re- 

 ceived special attention. Tricho])tera and Xeuroj^tera, on the other 

 hand, have lieen particularly neglected, and little has been done on 

 the abdominal musculature of Hemiptera, and of adult Lepidoptera 



