NO. 6 INSECT ABDOMEN — SNODGRASS 3 



morphology of the insect abdomen is a field in which no angel yet 

 has trod, and is, therefore, one in which the fool unhindered may 

 achieve his destiny. However, it is reputed that there is some merit 

 in knowing oneself to be a fool, and, if it is the wisdom of the wise 

 to enter only where the foolish have sprung the trap, the fool at least 

 has a mission to perform. Hence, the writer offers the generalizations 

 contained in this and a second paper to follow, not with the conviction 

 that they will prove infallible, but in the hope that others will try 

 to disprove them — and thereby establish their value. 



The principal conclusions derived from the study of the abdomen 

 and its appendages to be given later in detail may be itemized in 

 advance as follows : 



1. The insect abdomen consists primarily of 12 segments, the first 

 1 1 of which are true somites, while the last is the periproct, or telson. 



2. The twelfth segment is practically obliterated in all the true 

 Insecta, except for possible remnants in larval Odonata. 



3. The eleventh somite becomes the functional anal segment with 

 the suppression of the twelfth segment. Its tergum forms the epiproct. 

 The venter of the eleventh segment is distinct in some Thysanura, 

 but it is usually reduced or obliterated, except for two terminal lobes, 

 which are the paraprocts. The appendages of the eleventh segment 

 are the cerci. 



4. The tenth segment is usually distinct in generalized insects, Init 

 it is often combined with the eleventh to form a composite terminal 

 segment. The embryonic appendages of the tenth segment are sup- 

 pressed in postembryonic stages of the more generalized insects ; they 

 form the postpedes of holometabolous larvae, and perhaps the ap- 

 pendicular processes of the proctiger, or tenth segment, of some adult 

 Holometabola. 



5. The eighth and ninth somites are the genital segments, which 

 bear the gonopods. The median gonopore of the female is located 

 typically behind the eighth sternum, that of the male behind the ninth. 

 Deviations from these positions are secondary ; but the opening of 

 the paired oviducts of Ephemerida between the seventh and eightli 

 abdominal sterna probably represents a primitive condition, exhibited 

 also by the embryos of certain insects (see Heymons, 1892, and 

 Wheeler, 1893). 



6. The abdominal appendages of adult and larval insects are serially 

 homologous with the legs and mouth parts. Each consists of a basis, 

 and of one or two movable appendicular processes. The basis appears 

 to comprise the coxal and subcoxal regions of a typical ai)pendage, 

 the two parts being either distinct or united. The coxal part often 



