jvo. 6 INSECT ABDOMEN — SNODGRASS 17 



menoptera the entire first alxloniinal segment is so intimately united 

 with the metathorax that it forms anatomically a part of the thoracic 

 region of the hodv. ^lodihcation of the posterior visceral segments 

 will be noted in connection with a study of the genitalia. In females 

 of higher Diptera the functional visceral region is reduced to live 

 segments by the conversion of the posterior segments into a tubular 

 organ of ovi position. 



Tin-: gi-:nital segments 



The lateral ducts of the re])r()ductive organs in the majority of 

 the Hexapoda open into a common, median outlet tube. Exceptions 

 to this rule occur in the males of Protura, in both males and females 

 of Ephemerida. and in males of some Dermaptera, the two lateral 

 ducts in these cases o]>ening separately to the exterior. The ])osirion 

 of the genital a])ertures Aaries within the Hexapoda through an 

 extreme of six segments. In the Collembola the gonopores of 1:)oth 

 sexes are situated on the fifth al)dominal segment, while in the Pro- 

 tura thev occur l)etween the eleventh and twelfth segments. In the 

 Ephemerida the paired oviducts open between the seventh and eighth 

 abdominal segments, and the vasa deferentia open on the penes l)e- 

 tween the ninth and the tenth segments. In all other Pterygota, excc])t 

 Dermaptera, and in Thysanura and Dicellura. the single female aper- 

 ture lies between the eighth and ninth segments, and the male aperture 

 between the ninth and tenth segments. .Vpparent excei)tions to this 

 rule occur where some of the terminal segments are fused, where 

 one or more of the pregenital segments have been obliterated, or 

 where, as in female Lepido])tera, the gonopore has evidently under- 

 gone a secondary change in position. 



The genital apertures are described by some writers as being situated 

 on the segments, while others state that they occur between segments. 

 The gonopores, in truth, are probably located on the posterior parts of 

 the ventral surfaces of primary segmental areas, but since these ]mrts 

 of the primitive somites become the intersegmental membranes of tlie 

 definitive segments, the gonopores of adult insects are anatomically 

 intersegmental. They lie behind the primary sterna of the segments 

 on which thev are situated, and (jnly rarely is a secondary sclerotizatiou 

 formed behind them (male Odonata). The male gonopore is usually 

 carried (jutward on an evagination of the body wall forming a simple 

 penis, or it is situated on a more complex copulatory organ composed 

 of the ])enis and the i)arameres, or of the parameres alone, k'nown 

 as the lU'dciifiits. 



