NO. 14 INSECT ABDOMEN SNODGRASS IJ 



entia at its anterior end. The median penis of insects the writer 

 (1935) has designated the phallus, but the insect phallus is an intro- 

 mittent organ, whereas the median penis of most of the other arthro- 

 pods is not. A median ejaculatory duct is probably in some cases a 

 product of the union of paired ectodermal ducts (a ductus conjunctus) , 

 and in others an independently developed single invagination (ductus 

 communis) at the approximated orifices of the primary ducts. 



Another series of modifications in the development of unpaired 

 external genital structures begins with the formation of a wide, open 

 cavity of the ectoderm, into which the primary gonoducts discharge 

 (fig. 4 G, GC). The external pouch in this case is the bursa genitalis, 

 or genital chamber, and may be present in both the male and the 

 female. Its usually large external opening is the gonotreme (Gtr). The 

 mesodermal ducts (Vd) may open separately into the genital cham- 

 ber (Gprs), or their proximal parts may combine to form a ductus 

 conjunctus (H, Den) with a single gonopore (Gpr). This condition 

 is common in the Arachnida, where the ductus conjunctus is often 

 enlarged as an inner chamber, saccus internus (I, Si), of the exit 

 apparatus. An unusual development among the Arachnida occurs in 

 the Phalangida (J). The genital chamber (GC) here contains in each 

 sex a large organ evaginated from its inner wall, which serves as a 

 penis in the male {Pen) and as an ovipositor in the female. The 

 organ is traversed by a slender ejaculatory duct (Dej) beginning with 

 a bidbus ejaculatorius {Bej), into which opens the ductus conjunctus 

 (Den) formed by the united vasa deferentia. 



The bursa genitalis, or genital chamber, is of common occurrence 

 also in both sexes of the Chilopoda and Hexapoda. In these two 

 groups there is generally present in the male a single median penis, 

 which in its simplest form is merely a conical or tubular outgrowth 

 of the inner wall of the genital chamber (fig. 4K, Pen) traversed by 

 the ejaculatory duct {Dej) opening through the gonopore {Gpr) at 

 its extremity. The median penis, or phallus, of the insects, however, 

 may become a highly complex organ. It is often differentiated into a 

 proximal part, or phallobase (L, Phb), and a distal acdeagus (Aed). 

 The gonopore (Gpr) commonly becomes concealed within an end 

 chamber, or endophallus (Enph), and the external opening is then 

 the phallotreme (PJitr). In female insects the opening of the oz>i- 

 ductits communis (M, Ode), which is the true gonopore (Gpr), is 

 usually concealed in a large genital chamber (GC), the exposed ex- 

 ternal opening of which, or vulva, is the gonotreme (Gtr). The 

 genital chamber contains also the aperture of the spermatheca (Spt), 

 and is sometimes narrowed as a tubular vagina continuous with the 

 median oviduct. 



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