208 On Abdominal Appendages in Hexapoda. 



histological structure they show so many common features, 

 that we may, with H. Eisig *, think of them as repetitions of 

 old inherited tendencies. To this may be added that in the 

 pterygote Insects referred to they likewise occur during em- 

 bryonic life in a position relatively to the limbs which 

 corresponds with that demonstrated in the Thysanura and 

 Symphyla, inasmuch as the vesicular sac is always situated 

 within the coxal joint or the leg-like abdominal spur, just as 

 V. Graber has indicated in the development of Hydrophilus f. 



Now to glance at the physiological significance of the 

 ventral sacs, it seems probable, from the developmental 

 history of CEcanthus, and especially of the Cockchafer, as 

 already assumed by H. Ayers and V. Graber, that in these 

 Insects they perform secondarily a respiratory function, which 

 can only be regarded as a special development of cutaneous 

 respiration (the above-mentioned embryos, in the egg, lying 

 generally in moist earth), as the dorsal vessel and 

 tracheae are not yet in action when these ventral sacs possess 

 their highest development. That in the Symphyla and 

 Thysanura also the ventral membranous sacs have a similar 

 respiratory, and perhaps a specially excretory significance, is 

 supported by the defective or aborted development of the 

 tracheal system and the ventrally concealed position of the 

 stigmata in these forms. 



Thus Scolopendrella has only cephalic stigmata, the tracheae 

 from which extend exactly into the third segment, onward 

 from which the coxal membranous sacs occur. So also Cam- 

 podea has stigmata only on the three thoracic segments, and 

 these lead into feebly developed tracheae ; and Nicoletia f 

 according to Grassi, forms only delicate dorsal longitudinal 

 trunks, and feeble ventral transverse anastomoses, so that 

 here also the tracheal system appears to be only feebly deve- 

 loped. In Machilisy again, the longitudinal trunks are 

 entirely wanting, and the feeble abdominal tracheae present 

 only a slight ramification. According to the observations of 

 J. T. Oudemans (and the same thing was observed by the 

 author in the open) Macliilis in captivity extruded its ventral 

 sacs, especially if it were in a warm and at the same time 

 moist atmosphere, but always only when it was perfectly 

 quiet j this is against the one-sided conception of the ventral 

 sacs as defensive arrangements analogous to the fleshy forks of 

 the Papilionid larvae, for example, seeing that the latter come 

 into action only when their bearer is disquieted. 



* Monographie der Capitelliden &c. in Fauna und Flora von Neapel 

 &c. xvi. (1887), pp. 371-403. 



t Morphol. Jahrb. xiii. (1888), p. 60o. 



