24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



by a migration from the true sternal region of the last head segment, 

 which is the membranous floor of the neck behind the base of the 

 labium. 



The ventral muscles of the thorax retain apparently the primitive 

 condition in the larvae of most holometabolous insects. In the cater- 

 pillar, for example, the principal longitudinal ventral muscles consist 

 of two wide bands of fibers lying to each side of the ventral nerve 

 cord, extending through the entire length of the body, and attached 

 regularly on the intersegmental folds as are the dorsal longitudinals. 

 External to the dorsal and ventral intersegmental muscles of the cater- 

 pillar there is an intricate complex of small muscles disposed in all 

 directions against the wall of each segment. 



In certain larval forms, as in some Coleoptera, the attachment of the 

 ventral body muscles shows a condition intermediate between the 

 usual larval condition and that of the adult. In the larva of Dytiscus, 

 for example, as shown by Speyer (1922), though most of the ventral 

 thoracic muscles are intersegmental, being attached either to processes 

 of the intersegmental folds or to transverse ligaments arising from 

 the folds, some of the fibers extend between segmental furcal apo- 

 physes, which are present on each primary sternal region of the 

 thorax. The ventral muscle bands of the thorax are continued into 

 the abdomen, some of the fibers of the first abdominal segment being 

 attached anteriorly on the intersegmental fold behind the metathorax, 

 others on the furcal arms of the metasternum. In the adult of Dytiscus 

 (Bauer, 191 o) all the ventral muscles of the thorax are inter furcal in 

 their attachments, and none extends from the thorax into the abdomen. 



Ventral muscles from the thorax into the abdomen are absent in the 

 adult stage of many pterygote insects (fig. 35), though they may be 

 present in the larval or the nymphal stages. In the nymph of Psylla 

 mali, according to Weber (1929), two bundles of fibers diverge from 

 the base of the metaf urea to the anterior edge of the second abdominal 

 sternum, but these muscles, Weber says, are lost in the adult. 



In some insects, however, the ventral thoracico-abdominal muscles 

 are present in the adult stage. They are well developed in the cock- 

 roach (Blatta oricntalis), comprising here three pairs, the first arising 

 on the second spina, the second on a ligamentous bridge between the 

 bases of the metasternal apophyses, the third on the apophyses, all of 

 which are inserted posteriorly on the anterior margin of the second 

 abdominal sternum. The fibers arising on the metapophyses form the 

 anterior ends of the ventral longitudinal muscle bands of the abdomen. 

 In Gryllus, Voss (1905) describes a median pair of muscles arising 

 on the metafurca which branch posteriorly to the third, fourth, and 



