52 



HARDWICKE'S SCIEN CE -GO SSIP. 



Pectinibkanchiata. 



Bythinia tentaculata.—1\\\% snail suspends itself 

 by a thread, after floating, whicli is usually 

 attached to the surface of the water. 



Rissoa parva is well known to conchologists as a 

 thread-spinner. Mr. J. G. Jeffreys thus pleasantly 

 speaks of it :— "Lying on a rock, by the brink of a 

 seaweed-covered pool left by the receding tide, it is 

 no less pleasant than curious to watch this active 

 little creature go through its different exercises,— 

 creeping, floating, and spinning." 



Several other species of Rissose spin threads, also 

 Barleeia rubra, Eulima intermedia, CeritJmm reticu- 

 latum, Cerithlopsisttuhercularis, and Pleurotoma 

 nebula. An account of their different modes of pro- 

 cedure will be found in Mr. Jeffreys's work, under 

 theii" several headings. 



Litlopa, a genus of small mollusks living on 

 floating seaweed far from land, are said to use a 

 mucous filament for the purpose of regaining their 

 station, after having been swept off the weed.* If 

 this be correct, we have a water-breathing mollusk 

 using its thread as a means of ascent, after having 

 spun it downwards, a circumstance I have not 

 myself seen. My observation teaches me that these 

 threads are not used by mollusks against the laws 

 of gravitation. 



With regard to the spinning of Neritina fliivia- 

 tUis. This species is an inhabitant of running 

 streams, and will not live long in confinement. Its 

 structure renders it impossible for it to spin an 

 . upward thread, as the nature of its habitat alike 

 precludes it, and as it could not float in running 

 water, it could not therefore spin a doiomcard thread, 

 . as obtains with other members of its order. "While 

 making these observations, I do not discredit Mr. 

 Warington's statement, because, although the act of 

 floating is not a normal one with the creature, it 

 might have performed it as mollusks sometimes 

 do,t when placed under circumstances which allow 

 of it, albeit in their natural condition they could not 

 possibly do it ; and if it floated, there is no reason 

 why it should not have spun a downward thread. 



Having kept nearly every British species of the 

 Limnseidfe in confinement on purpose to observe 

 their habit of spinning, and not having seen some 

 species use this means of locomotion at all, others 

 seldom, and some often ; some when young but less 

 often as they grow older, and others all their life- 

 time, I have been led to advance a theory whereby 

 to account for this varied use of these threads. To 

 this end I have drawn up the following table. While 

 writing it, I am sensible of its imperfections ; but if 

 it only serves as a nucleus to stimulate other ob- 



servers of the economy of these creatures to frame 

 a more] perfect one, I shall be the more satisfied 

 with my attempt. 



I must ask the reader to bear in mind that the 

 time and opportunities at my command for ob- 

 serving their life and habits do not admit of my 

 coming to the conclusion, that, because I have never 

 seen a species spin a thread, therefore it does 

 not do so. On the contrary, I believe that all the 

 Limnajidffi use this method of travelling, more or 

 less ; and, doubtless, in their native habitat, when the 

 eye of man is not present to pry into their secrets, 

 these seemingly insignificant creatures perform 

 these their appointed acts, while we, most wishing 

 to see, see them not : — 



Flanorbis /i«m^«s.t— Inhabiting streams ; could 

 not spin a thread in its native habitat, 1 have not 

 succeeded in keeping it alive long. 



Flanorbis nitidus,f P. naiitileus,-\ P. albus,-\ P. 

 glaber,-\ P.vortex,-\ P. spirorbis,*P. contortus,* Lini- 

 naa truncatula.-\—Oi these species, some spend 

 their lives on vegetation near the surface of ponds 

 or pools, and others inhabit shallow ponds or 

 ditches, which sometimes become dry in summer ; 

 hence the necessity for using a thread does not 

 often occur. 



Planorbis carinatus,* P. complanatus.^ — Living 

 in the larger ponds and pools where the water is of 

 considerable depth, this capability of thread-spin- 

 ning often serves them to good purpose. 



Physa hypttorum,*P.fontinalis,* Limncea glabra.^ 

 —Inhabiting deep ditches, ponds, or pools, and fond 

 of indulging in subaqueous excursions, the habit of 

 spinning is essential to their mode of life. 



Limntsa stagmlis,* L. palustris,* L. auricularia,-\ 

 L. peregra,% L. glutinosa,X Planorbis corneiis.-\— 

 When full grown, these species, being much larger 

 and stronger than any of the foregoing, are able to 

 traverse more ground in a given time ; hence they 

 do not feel the necessity .of using a thread so 

 often as the smaller species. 



* Johnston, " Introduction to Conchnlofy," p. 134. 

 ir For an account of this habit in Trochiis occidentalis, a 

 ■deep-sea species, see Jeffres'S, " Brit. Con.," vol. iii. pp. 335-6. 



HEMIPTERA. 



(Continued.) 



HoMOrTERA.— The EEOG-HOPrEE [ApJirophora 

 spumaria). 



IN last October number of Science-Gossip I 

 drew the attention of its readers to the ovipo- 

 sitor saws and suctorial tubes of the Nettle-bug 

 {Stenocephalus agilis), order Heteroptera, or " dif- 

 ferent wings." 



From communications received, and numerous 

 applications for mounted slides of ovipositors, from 



* Species I have seen spin a thread. 



t Species I have Icept, but not seen spin. 



X Species seen to spin by others. 



