NO. 2 THORACIC MECHANISM OF A GRASSHOPPER — SNODGRASS 5 1 



III. THE THORACIC MUSCLES OF DISSOSTEIRA 



The evolution of insect structure has been largely an evolution of 

 mechanisms made up of the cuticula and the muscles. Though the 

 study of the insect skeleton will remain the most important branch of 

 insect anatomy for purposes of taxonomic description, it is becoming 

 evident that the morphology of the skeleton is not to be understood 

 without a knowledge of the relations that exist between the cuticular 

 modifications and the muscles. Systematists and anatomists have con- 

 sumed much time and have occupied much printed space with discus- 

 sions of homologies between sclerites, which, in many cases, are of 

 little value because the fundamental structure of the parts in question 

 has not been studied and because mechanical relationships have been 

 entirely ignored. The time is at hand when we must understand in- 

 sects as living creatures rather than as museum specimens. Alorphol- 

 ogy must become a basis for the study of function, including both 

 the physiological processes by which the insect is maintained as a liv- 

 ing thing, and the mechanisms by which it directs its bodily activities. 



A grasshopper furnishes a particularly good subject for the study 

 of insect musculature. Not only are the individual muscles easily dis- 

 tinguished in dissections, but the muscles present are principally those 

 that are common to all generalized insects. Fresh specimens do not 

 serve well for the purpose of muscle study, but after twenty-four 

 hours' immersion in 8o per cent alcohol the fiber bundles become more 

 compact and are more readily seen as separate muscles. Since most 

 of the insect's muscles are arranged laterally, a median sagittal section 

 of the body will give the best approach to the muscles for an initial 

 examination ; but eventually it will be necessary to cut specimens into 

 numerous pieces, for each muscle must be followed from one attach- 

 ment to the other. Never accept a supposed observation for a fact 

 until it is seen alike in at least two preparations — not that specimens 

 differ, but that observations frequently do. 



It is customary in describing muscles to follow them from their 

 origins (fixed ends) to their insertions (movable ends), but the mus- 

 cles of insects are in general more easily studied by finding the inser- 

 tion points first and then tracing the bundles of fibers out to their 

 basal attachments. The origins of muscles are likely to vary more in 

 different segments and in dift'erent species than are the insertions, and 

 branched muscles are often confusing until their common parts or 

 apodemes of insertion are determined. 



The student will find that the principal thoracic muscles of Dis- 

 sosteira more nearly correspond with the description of the muscles 

 of the field cricket, Liogryllus {Achcta) campestris, given by Carpen- 



