32 ILLINOIS BIOLOGICAL MONOGRAPHS [350 



ber of annulets. In the specialized Tenthredinoidea the number varies 

 from one to seven, but five to seven is of the most common occurrence. 

 The number becomes smaller in the highly specialized families, being 

 reduced to a single annulet in the Siricidae and Orysidae. 



Spiracles. — The spiracles {spi) are present on the prothorax and the 

 first eight abdominal segments in all Tenthredinoidea. The pair on the 

 prothorax are always the largest. The abdominal spiracles are usually 

 uniform in size and shape except the last pair, which are often larger than 

 the others. The spiracles are definite in location in regard to the annulets 

 and are always situated on some one of the first three annulets of the 

 segments. The spiracular line is usually located slightly ventrad of the 

 middle of the lateral aspect of the body, but sometimes it migrates ven- 

 trad to the latero-ventral line as in Caliroa. The spiracles (Fig. 155) are 

 usually very simple in structure, vertical in position, never circular in 

 outline but narrowly ovate, rounded or pointed at both ends. The peri- 

 treme is narrow but strongly chitinized and brownish or blackish. The 

 labiae are narrow and the spiracular opening is usually closed and appears 

 like a dark line. The peritreme is sometimes distinctly thickened as in the 

 Hylotominae. There is often a semicircular or irregular chitinized colored 

 area on each side of the spiracles, as in certain nematid genera. These 

 areas vary in size and shape but are constant within species, and their 

 presence is usually constant within genera and often within a subfamily. 

 When these areas are present, the spiracles are said to be winged. 



The true prothoracic spiracles are considered as wanting in adult and 

 larval insects. The spiracles found on the prothorax of Tenthredinoid 

 larvae are the mesothoracic spiracles {msp) which have migrated from 

 the mesocoria onto the prothorax. The metathoracic pair {tsp) is usually 

 functionless, very small, and located in the metacoria, or obsolete. In the 

 Cephidae and Siricidae, however, the metaspiracles are distinct, functional, 

 and as large as the abdominal spiracles. It is difiicult to explain the 

 rudimentary condition of this pair in the Xiphydriidae, since other charac- 

 ters indicate that they have a common origin with the Cephidae and 

 Siricidae. It is possible, however, in the course of evolution, to have one 

 structure of the body modified faster than another structure. 



Setae. — The surface of the body is usually provided with some setae, 

 particularly on the head, thoracic legs, and the ultimate segment of the 

 abdomen on the suranal and subanal lobes. The number, size, arrange- 

 ment, and structure of the setae vary in different taxonomic units, accord- 

 ing to their location; and to some extent according to the stage of larval 

 growth. There is a tendency toward the loss of setae in the ultimate 

 stage or the last instar, as is Pteronidea, or to have fewer and smaller setae 

 in the leaf-mining and wood-boring larvae, as in the Fenusinae, Cephidae, 

 and others. There seem to be no definite setal patters, as in lepidopterous 



