42 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



The musculature of the thoracic spiracles is usually dift'erent from 

 that of the abdominal spiracles, as is the structure of the spiracles 

 themselves. The spiracles of Apterygota and Ephemerida are said 

 to have no musculature. 



THE ABDOMINAL MUSCULATURE OF ADULT PTERYGOTA 



The musculature of the visceral segments of the abdomen in 

 pterygote insects adheres closely to the generalized plan of structure, 

 though there are usually slight aberrations in the first segment or 

 first and second segments. The musculature of the genital and post- 

 genital segments is often higlily specialized or reduced, but it is un- 

 doubtedly derived from the same muscle pattern as that prevailing 

 in the less modified segments. The usual departures frtjm the gener- 

 alized musculature in the visceral region of the abdomen consist 

 principally of a reduction in the number of muscles, a shortening in 

 the length of some of them, and a shifting of the points of attach- 

 ment, bringing about simple changes in the position of certain mus- 

 cles. A brief examination of the orders in which the abdominal mus- 

 culature is best known will serve to show the extent and nature of the 

 modifications that take place in the visceral segments. The more ex- 

 tensive modifications in the specialized genital and postgenital seg- 

 ments need not concern us here. 



Ephemerida. — The most generalized abdominal musculature of the 

 adult pterygote type occurs in the Ephemerida, and the muscle pattern 

 is here essentially the same in both adult and larval stages. We may, 

 therefore, follow Diirken's account of the larval musculature of 

 Ephemerella ignita, which can easily be verified in any epliemerid 

 species. Most of the abdominal muscles (fig. 15 A), except those in- 

 serted on the gill bases (bmcls), lie in a single plane against the body 

 wall, and are comprised in dorsal (d), lateral (/), and ventral (v) 

 groups. The first two dorsals (id. 2d), counting outward from the 

 median line, and the second and third ventrals {2v, jv) in most of 

 the segments are typical intersegmental, longitudinal muscles attached 

 on the anterior margins of successive segmental plates. The third 

 dorsal (3d), however, is atypical in that most of its fibers take their 

 origin on the middle of the tergum and cross the following segment 

 to be inserted on the anterior margin of the second tergum follow- 

 ing. This muscle Diirken calls a " compound intersegmental " mus- 

 cle. The fourth dorsal ( /</) and the first and fourth ventrals (iv. 

 4v) are short muscles arising on the posterior parts of the segmental 

 plates before those of their insertions. These muscles thus appear 



