NO. O INSECT ABDOMEN SNODGRASS 27 



posed to have any relation to the egg-laying organ of the true Insecta, 

 since it arises from between the eleventh and twelfth segments of the 

 abdomen. No structure homologous with the insect ovipositor occurs 

 in any arthropods other than the insects. 



The thysanuran ovipositor is composed of four processes arising 

 from the stylus-bearing lobes of the eighth and ninth abdominal seg- 

 ments. These lobes, commonly designated " plates ", are generally 

 conceded to be the bases (" coxites ") of true abdominal Hmbs. The 

 pterygote ovipositor differs from that of the Thysanura only in its 

 greater complexity ; an essential part of its structure consists of four 

 basal lobes, or plates, that appear without question to be direct homo- 

 logues of the bases of the thysanuran gonopods. Hence, on the evi- 

 dence of the limb origin of the component parts of the ovipositor in 

 Thysanura, the parts of the pterygote ovipositor must have had the 

 same origin. Considering, therefore, the many respects in which the 

 Thysanura, particularly the Lepismatidae, show that they are more 

 closely related to the Pterygota than are any other of the apterygote 

 forms, we may reasonably assume that the ovipositor had its inception 

 in the common ancestors of these two groups of insects, and that it 

 has been formed by special modifications of the appendages of the 

 eighth and ninth abdominal segments. 



The Machilidae are in many respects more primitive in their struc- 

 ture than are the Lepismatidae ; the structure of the abdomen in 

 Machilidae is more generalized even than in any other family of the 

 Apterygota. The relatively primitive condition of the machilid abdo- 

 men is shown particularly in the well-known fact that the bases of the 

 appendages, throughout the visceral and genital regions, are distinct 

 from the small median sterna. The abdominal limb bases form a 

 double series of large, stylus-bearing plates intervening between the 

 terga and the sterna, though the two plates of each segmental pair 

 are usually united with each other in the median line posterior to the 

 sternum. Each so-called " plate ", however, is really the ventral wall 

 of a flattened lobe, the membranous dorsal wall of which is reflected 

 into the intersegmental membrane between its segment and the seg- 

 ment following. The stylus is borne in a notch on the distal apical 

 margin of the lobe, and the stylus muscles arise on the anterior margin 

 of the ventral plate. Mesad of the stylus the limb base lobe of most 

 of the pregenital appendages bears on its distal margin also one or 

 two retractile and eversible vesicles, the muscles of which take their 

 origins on the ventral plate. Vesicles are never present on the 

 gonopods. 



That the stylus-bearing plates of the Machilidae are the bases of 

 the abdominal appendages is here taken for granted, and is made the 



