370 MR. WALTER WESCHE ON THE GENITALIA OF 



that develop on all parts of insects : under the first law, though admitting infinite 

 variety of form, those forms are rigidly confined within the limits of a fixed scheme 

 and can he homologized throughout the Insecta ; under the second law, though 

 admitting an even greater variety of form and contrivance (all, it is true, with a very 

 ohvious objective), the forms are not capable of homologization ; we can recognize that 

 an identical cause has broadened and shortened the tarsi of Platychirus and Di/Hscus, 

 but the extraordinary suckers on the tarsi of the Beetle can only be recognized as 

 homologous in its family ; they have no counterpart in the fundamental scheme of 

 Insecta. 



A similar law, governed by a different necessity, has developed the " ptilinum " in 

 both sexes of the Cyclorrliaplia in Diptera. 



If we could adopt the old idea of design in Nature, we might think of the genitalia 

 and mouth-parts as formed on the same plan, or as the working-out of the same idea. 

 Palling that, we assume that they might be derived from organs W'hich, placed at both 

 extremities of the organism, shaped themselves by unknown but similar laws of growtii. 

 So we find the extremities furnished with hamate appendages such as maxillse or 

 forcipes interiores, and sensory appendages such as mamillary or genital palpi, and these 

 appendages surrounding and attached to a central organ which itself has special sense- 

 organs attached to it, " taste-hairs " on the labium, or the aculeate membrane on the 

 penis. 



I submit that if it were a matter of development along the " lines of least resistance," 

 or of growth similar to the second law, we might expect frequent departures from the 

 unity of plan that we can trace in these organs ; that possibly appendages fulfilling 

 the functions of mouth-parts would be found attached to the fore legs, if not in most, at 

 least in some species, as in that position they could be used with equal, if not greater, 

 convenience. This is contrary to all expei'ience, but when we leave the true insects, 

 and examine such a group as the Arachnida, we notice the absence of the unity of 

 design both in the mouth-parts and the genitalia. 



Genilalia in the Arachnida and Hydrachnidcp. — In the Spiders the males have the 

 genitalia on two palpiform processes which spring from the roots of the fore legs, as in 

 Meta seymentata and Stemonijphantes lineatm, and these vary from extreme simplicity 

 to extreme complexity ; while in the AVater-Mites a spermatophore is picked out of the 

 genital pouch by the claws of one pair of legs and transferred to the genital pouch of 

 the female, both organs having a similar situation on the centre of the abdomen. 



In Odonata. — It may be argued that even in the true insects there is a departure 

 from the general type in the Odonata, where the copulatory apparatus is in a segment 

 far removed from the extremity of the abdomen, though the forcipes and the genital 

 pore remain in their usual position. I have made only one or two dissections in this 

 group, and these are not complete, but my preparations show a theca, genital palpi, 

 forcipes interiores, a penis, and a spine, which can be homologized (putting aside the 

 segmentation) with the corresponding parts in Diptera ; this is in ^schna cyanea, Miill. 



In Orthoptera. — Some of the Orthoptera also present a difficulty, as in many species 

 a spermatopliore is transferred by a hook to the cloaca of the female ; this is, according 



