30 PARTHENOGENESIS IN SOME 



is really due to an interruption, which the walls of the sac ex- 

 hibit at this point in almost the whole of their transverse diameter. 

 The body of the caterpillar is indeed also spirally curled (figs. 4 

 and 7)> but in its form and length it only corresponds with the 

 lowermost whorl of the sac. In this way it would be impossible 

 for the caterpillar to push its body up into the uppermost narrow 

 whorl for the evacuation of its faeces. The female of Psyche 

 Helix, like all females of Psyche, after completing the business 

 of oviposition, quits its sac, which is firmly spun down by its 

 anterior aperture, and for this purpose it makes use of the lateral 



still furnished with opercula. These consisted of a dried pupa, which in the 

 form of the legs and of the long antennae, the four hairy rudiments of wings, 

 and the two biting jaws, exactly resembled a Phryganidan. The description 

 given by Lea of his Valvata arenifera (in his Observations on Najades and 

 Descriptions of new species, vid. Transactions of the American Philosophical 

 Society, vol. iv. Philadelphia, 1834, p. 104. pi. 15. fig. 36 a, b. See my copies, 

 figs. 23, 24) runs as follows : " Testa orbiculata, convexa ; anfractibus tribus, 

 qui arenis agglutinatis operiuntur; umbilico lato; spira obtusa. Hab. Cum- 

 berland River near Nashville. Length four-twentieths of an inch. Remarks. 

 This very curious and interesting species was among the freshwater shells so 

 disinterestedly sent to me by the Lyceum of Natural History of New York, to 

 be examined and inserted in this paper. It has the singular property of 

 strengthening its whorls by the agglutination of particles of sand, &c., by 

 which it is entirely covered, and in this character it resembles the Trochus 

 agglutinans, Lam. (Trochus conchy liophorus, Authors). The apex, in all the 

 specimens which I have had an opportunity of examining, is broken. The 

 operculum was observed in two specimens sufficiently perfect to exhibit a 

 striated horny structure." 



The sacs of Helicopsyche minima communicated to me by Bremi, agree 

 almost perfectly with this shell of Valvata arenifera described and figured by 

 Lea. Even the bronze-green colour is common to both of them. The pre- 

 sence of an operculum is also in favour of the derivation of this habitation 

 from a Phryganidous insect, as the Sac-bearers amongst the Lepidoptera form 

 no operculum, but always spin down their sac by its lower aperture to foreign 

 substances. Moreover, the opercula, of which I found several in my specimens 

 of the sac of Helicopsyche minima, had also a striated appearance like those of 

 Valvata arenifera. They were smaller than the aperture of the sac, and con- 

 sequently only closed it imperfectly. On examining them with the micro- 

 scope, I detected a fibrous structure in these opercula, arising from compara- 

 tively coarse-spun threads, sticking close together; at the margins of these 

 opercula single threads protruded, by which they were united with the mouth 

 of the sac. In my specimens of the sacs of Helicopsyche Shuttleworthi I per- 

 ceived no opercula; they had probably fallen off, or perhaps were not formed 

 when these sacs were collected. 



