NO. 6 INSECT ABDOMEN — SNODGRASS 4I 



first abdominal segment, but also in the mesothorax and the meta- 

 thorax (Brocher, 1922). In the larvae of anisopterous Odonata, 

 according to Whedon ( 1919), a muscular dorsal diaphragm is present 

 only in the fourth or the fifth segment of the abdomen. 



The ventral transverse muscles in some of the Orthoptera (Tet- 

 tigoniidae and most of the Gryllidae) take the form of widely sepa- 

 rated compact bundles of fibers crossing the anterior parts of the 

 segmental sterna. In others, as in the Acrididae, the origins of the 

 fibers are distributed along the sides of the sterna, and the muscles 

 form a typical ventral diaphragm occupying most of the length of the 

 abdomen. Ford (1923) thinks that the compact type, that is, the one 

 in which the fibers form individual transverse muscles segmentally 

 arranged, is the primitive type, and that it has been derived from a 

 difi^use or web-like type. The writer, however, believes that the 

 relations may be the reverse, especially considering Heymons' 

 (1895) statement that the transverse muscles of Orthoptera are 

 formed in the embryo along the intersegmental folds. In the larvae 

 of anisopterous Odonata, according to Whedon (1919), there is 

 in the abdomen only a single, large, spindle-shaped, somewhat flat- 

 tened ventral transverse muscle lying in the extreme anterior part 

 of the sixth segment, attached laterally on the intersegmental fold. 



In the higher insects in which ventral transverse muscles are present 

 in the adult, as in Hymenoptera, the muscles form a continuous sheet 

 of tissue over the ventral sinus. Ventral transverse muscles are usu- 

 ally absent in holometabolous larvae. In the honeybee larva, Nelson 

 (1924) says, there is present in the newly hatched larva a well- 

 developed ventral diaphragm consisting of a continuous sheet of trans- 

 verse fibers, but in older larvae it becomes a vestigial structure formed 

 of more or less isolated fibers entirely too few in numbers to con- 

 stitute more than a loose and insignificant meshwork. 



The spiracular muscles. — The musculature of the abdominal spira- 

 cles includes one or two muscles associated with each spiracle. The 

 muscle most generally present is an occlusor. This is a short muscle 

 usually attached at both ends on the base of the spiracular atrium, 

 where its contraction compresses the inner end of the atrium and so 

 closes the entrance to the trachea. In the Acrididae the occlusor 

 muscle arises dorsally on the tergal wall close behind the spiracle. A 

 dilator, or opening muscle of the spiracle, antagonistic to the occlusor, 

 occurs at least in most of the Orthoptera, Lepidoptera, and Hymenop- 

 tera, but it is absent in Odonata, some Orthoptera, and Coleoptera. 

 The dilator commonly takes its origin on the tergum or on the lateral 

 margin of the sternum of the segment in which the spiracle is situated. 



