NO. 14 INSECT ABDOMEN SNODGRASS I9 



in Xiphosurida, Malacostraca, and Diplopoda, or where in these same 

 groups there may be a single median penis, either the eggs are insemi- 

 nated outside the body of the female, or the sperm are transferred to 

 the female receptaculum by s])ecially modified appendages (gonopods). 

 In the Malacostraca the intromittent organs are the appendages of the 

 first and second abdominal segments, so modified as to form organs 

 for receiving the sperm or spermatophores from the penes or penis, 

 and for inserting them into the oviducts or sperm receptacles of the 

 female. Similarly in the proterandrious Diplopoda the eighth, or the 

 eighth and ninth, pairs of legs are modified for intromittent purposes, 

 the genital outlets being situated on a pair of penes, or a median penis, 

 on the segment of the second pair of legs. Among the Arachnida the 

 intromittent organs are the chelicerae in Solpugida, the pedipalps in 

 Araneida. The males of opisthandrious Diplopoda are said to use the 

 mandibles for transferring the sperm to the female. A nonappendicu- 

 lar intromittent organ not associated with the genital aperture occurs 

 in the insect order Odonata. This organ is a large, median, penislike 

 structure on the under surface of the second abdominal segment of 

 the male, which receives the sperm from the genital opening on the 

 ninth segment and is the agent of transfer to the female during 

 copulation. 



The median penis becomes the functional intromittent organ prin- 

 cipally in the Chilopoda and the Hexapoda, though the mating habits 

 of entomostracan Crustacea having a median penis are not well known, 

 and a prominent exception among the arachnids occurs in the Phalan- 

 gida, in which the sperm are transferred to the female through a large 

 unpaired organ containing the genital exit duct. The penis of the 

 chilopods is never highly developed, and appears to be merely a fold 

 or conical outgrowth of the body wall around the genital aperture. 

 The median penis (phallus) of the insects may be a small simple 

 structure as in the chilopods, but more usually it has a tubular form, 

 and may attain a great complexity of structure. Students of insect 

 morphology have often regarded the insect penis as having some rela- 

 tion to the gonopods of the male genital segment (ninth abdominal), 

 the organ being supposed to be formed by the union of branches or 

 lobes from the bases of these appendages. The ontogenetic develop- 

 ment of the insect penis, however, as will be shown in a later part of 

 this paper, does not support this view, and considering that the analo- 

 gous organs in other groups of arthropods are mere integumental 

 outgrowths, there seems to be no reason for assuming the organ has 

 any other morphological status in the insects. 



