Imagens das páginas
PDF
ePub

MOLLUSCAN THREADS.

By G. SHERRIFF TYE.

ONTAGU, at the beginning of this Mr. Reeve does not tell us how he proved his

century, noticed the habit in Physa assertion about the “hydraulic action of the foot,” fontinalis of thread-spinning. He and does not seem to have tried to ascertain how says : Physa fontinalis spins a | they “ hold themselves stationary at different depths filament by which it lets itself in the water,"—coolly "doubts" Montagu's statedown from the surface after float ment about the "mucous thread,” and does not ing.” Later, Mr. Robert Waring. notice Mr. Warington's observations at all. I may ton* gave an exceedingly interest state that a mollusk is only capable of "holding ing account of this thread-spinning itself stationary at different depths in the water » by Limnæa glutinosa, L. stagnalis, when attached to a thread, and that no "hydraulic

various species of Planorbis (not action ” of the foot takes place. When a mollusk named by him), and Physa fontinalis. is forming a thread, the “lateral margins ” of the

The latter upon one occasion formed foot are brought together, forming a channel for the a thread so tough that he was enabled natural flow of mucus down the sides of the foot to lift the snail seven inches above to the tail; thus adding to the thread, which is the surface of the water by it. The gradually extended. The existence of a thread may

author includes in his list of thread. be proved, as stated by Mr. Warington, by passing spinners Neritina fluviatilis-of this I shall a rod under the creature, by which means it can be

speak further on-and concludes by swayed to and fro. stating his belief that “all the fresh-water snails I have taken great interest in this thread-spin. are possessed of this power."

ning, and long before I had read Mr. Warington's Now, after this well-proven fact of spinning,

excellent notes I had been observing this seeming stated upon the authority of so good an observer,

phenomenon, and had tabulated the species absoyou would scarcely expect to find such an observa

lutely seen by myself in the act, and noted the tion as this :-“The Physæ, especially P. hypnorum,

conditions under which mollusks are capable of are active in habit, whether swimming foot upper

producing and using a thread. most, on the surface of the water, holding them

Let me here explain that the words thread and selves stationary at different depths in the water, spinning are used descriptively, and it must not be or gliding through it in sudden jerks by an hydraulic supposed that these threads, or the production of action of the foot. By bringing the lateral margins then, bear any analogy to the spinning of spiders. of this organ into contact, the animal constructs a

In the case of the mollusk the thread is gelatinoustube for inhaling and suddenly expelling the water

in fact, is formed of the slime of the creature, the either upwards or downwards. Montagu stated, and process of forming it being, to a certain extent, an the statement has been repeated by Jeffreys, that involuntary act, although it is used for a set the animal spins a mucous thread for letting itself purpose; whereas the spider's thread is silken, and down in the water and rising again for respiration;

its formation is entirely under the control of the but I have not succeeded in confirming this observa.

creature. Neither are they to be confounded with tion, and have great doubts of its accuracy.”+

the byssal filaments of the Mytilide, Pectinida, Dreissena polymorpha, &c., these latter being of a

fibrous nature, and the product of a special organ. * Zoologist, 1852, pp. 3634-5; 1855, p. 4533. † Lovell Reeve, “British Land and Fresh-water Mollusks,"

As members of the order Pulmonobranchiata, pp. 150-1. 1853.

breathers of atmospheric air-spin and use threads No. 111.

[graphic]

D

oftener than any other of the Gasteropoda, owing to some difficulty in constructing a vessel of
especially the aquatic members of the group, and as such a depth convenient for observation, I have not
their method of using them differs from the Pectini been able to verify my belief.
branchiata-water-breathers—we will consider them Permanent threads are kept in position and strong
first.

enough for use by the addition of a film of mucus
In order to be better understood, let us describe each time a mollusk crawls over them; and I may
briefly their process of respiration. On the side of here explain what I wish to convey by saying that
the creature is situated a sac, or branchial chamber, / the process of spinning is to a certain extent an
formed by a fold in the mantle, and having an involuntary act.
opening outwards, which the animal can open and When a snail crawls (either a terrestrial or an
shut at will. The air in this sac is renewed by aquatic species) it leaves behind it a trail of mucus,
diffusion while the mollusk is at the surface of the which is discharged for the purpose of lubricating
water, which air oxygenates the blood through the the foot in its passage over any surface, and if the
veins, which ramify in an arborescent form over the continuity of this mucus be not ruptured, we have
roof of the cavity. Now it will be obvious to the a thread in all respects analogous to those I am
reader that when this sac is distended with air, the speaking of.
creature becomes of less specific gravity than water; In the case of an aquatic species, this trail of
hence it will float, even against its own will, when mucus is usually invisible; hence it may be sup-
dislodged from its hold; and, on the other hand, posed that mollusks inhabiting water do not secrete
when the air in its branchial chamber is exhausted such a copious supply as their brethren of the land,
by natural respiration, or expelled by reason of some and that the water itself would act as a sufficient
annoyance, the creature, becoming heavier than lubricant; but such is not the case, for not only do
water, at once sinks to the bottom; and on this the bodies of mollusks require lubricating in their
simple fact hangs the capability of the mollusk to passage through water (as in the case of fishes), but
spin an upward or downward thread.

the foot especially, in its passage over the surface I have never seen a member of this order descend

of any object. This mucus may readily be seen by a thread unless it had first ascended by one, in when fresh water is put into any vessel in which which case it might return upon the same thread. mollusks have been kept for a few days, as the It would no doubt be possible for it to descend by bubbles of oxygen then given off by the plants a thread if its air-chamber was sufficiently empty (Anacharis alsinastrum shows it well) adhere to the to allow of its sinking; but, atmospheric air being network of mucus which stretches from leaf to essential to the creature's existence, it very rarely leaf, making it plainly visible : of course the change voluntarily descends without a supply, and never in must be conducted gently. The best plan is to lift such a case by a thread, although it will creep out a bundle of Anacharis from the vessel in which about in the water when the air in its branchial the snails are, and drop it gently into a vessel of cavity is sufficiently exhausted to allow it to fall fresh water. to the bottom of the water when loosed from its The slugs possess this mucus-secreting prohold.

perty to a remarkable degree; each species produces As soon as a young Limnæid issues from the egg mucus of a colour and consistency peculiar to it appears to be capable of rising to the surface of itself, some species being provided with an imthe water by a thread, its air-sac being no doubt portant slime-gland near the tail. This property sufficiently charged with air to render it buoyant is essential to their well-being ; having no shelterenough.

ing shell, it serves to keep their body moist and The method of anchoring these threads to the cool in dry weather. Slugs often suspend themsurface of water is singular: a minute concavity at selves by a thread, but do not use it as a means of the upper end acts like a small boat-of air, and ascent. The Pectinibranchs, extracting oxygen thus sustains the thread.

from the water as it passes over their comb-like When one of these mollusks descends by the gill, are not capable of altering their specific thread it spun in ascending, it generally carries gravity; hence they cannot spin an upward thread; back the thread with it, gathering it together by a but several species, both fluviatile and marine, muscular action of the foot, although these threads often suspend themselves from the surface of the are sometimes fixed and made to last a considerable water or from a floating object, by a thread, but do time. The longest threads I have seen are those of not ascend by it again. The same remarks apply the Physæ, and I have had in a vessel containing to the Nudibranchs. fourteen inches depth of water, a number of them Instances of thread-spinning occur among the fixed by Physa hypnorum, up and down which they Lamellibranchiate mollusca. Sphærium lacustre were creeping for eighteen or twenty days together. has been observed by the late Dr. Lukis, of I have no doubt they can extend their threads to a Guernsey, to suspend itself below the surface of much greater length, say three or four feet; but, the water by a filament half an inch in length, the

[merged small][ocr errors]

nours.

spinning of which occupied the creature three can, and the mancuvres they go through upon

M. Bouchard-Chantereux has recorded their fairy ladders outdo the cleverest human gymthat the young of S. corneum possesses the same nast that ever performed. I once saw one ascendpower of spinning a thread. I have myself seen ing, and when it was halfway up the thread it was the latter anchor itself by a mucous filament. The overtaken by another; then came the “tug of uses of these threads to the Pulmonobranchs appear war"; each tried to shake the other off, by repeated to be :

blows and jerks of its shell, at the same time creep1st. They enable the mollusk to reach the ing over each other's shell and body in the most surface of the water gently when no other means excited manner. Neither being able to gain the present themselves.

mastery, one began to descend, followed by the 2nd. It is a much easier method of locomotion. other, which overtook it, reaching the bottom

3rd. It is a much quicker mode of travelling; first. Yet they are not always bent upon war, but for if the surface traversed be smooth, as the pass and repass each other in an amicable spirit. side of a glass vessel, it will take the mollusk One of the most beautiful sights in molluscan twice the time to creep as to float by a thread, economy is to see these little “ golden pippins” while if the surface be uneven, as the side of a gliding through the water by no visible means; and pond or the leaves of a plant, it would be longer when they fight, to see them twist and twirl, perstill in creeping.

forming such quick and curious evolutions, while 4th. As a great part of the lifetime of the Lim seemingly floating' in mid-water, is astonishing, næidæ, especially the Physæ, is spent in floating even to the patient student of Nature's wonders. upon the surface of the water, where they feed upon Physa fontinalis stands next as a thread-spinner, particles of decaying vegetable matter, this property using the thread in a similar manner, but not so often. of thread-spinning seems admirably suited to their Limnæa glabra, although not using this means of requirements.

locomotion so often, nevertheless spins well and It enables the slugs to descend from considerable easily. heights, as from branch to branch of a tree, quicker L. stagnalis is active when young, but its habit of and easier than by the process of creeping.

spinning decreases as it grows older. Among the Pectinibranchs, it enables the snail to L. palustris.-The same remarks apply to this reach the bottom gently, instead of falling roughly species also, although I have not seen it spin so or suddenly. It serves the same purpose among commonly as stagnalis. the Nudibranchs.*

L. peregra.- This species has been observed to The Sphæridæ, through their capability of climb. spin by my friend Mr. R. M. Lloyd, but it very ing and floating, in which exercises they are fond seldom uses a thread. of indulging, especially when young, are enabled L. glutinosa, recorded as a thread-spinner by Mr. to enjoy a more extended range of habitat and food; Warington. and when, during their excursions, they desire to Planorbis complanatus, P. spirorbis, P. conrest, this mucus-cable (always short, generally tortus.—These species spin very much less often hardly to be spoken of as of any length, but simply than the . a mucous attachment) keeps them safely moored, Limax arborum.-M. Bouchard-Chantereux has while, with foot and siphons withdrawn, they take seen young individuals of this species descend from a short period of repose.

branch to branch of a tree by a mucous filament, Having thus far, I hope, succeeded in indicating and he supposes this species to be the Limax filans, the "why and wherefore ” of molluscan threads, or spinning slug of some English authors of the I will tabulate the species I have seen spin, and

last century. Mr. Daniel has also seen this species those seen by others, commencing with the species suspended in couples from the branches of trees that spins oftenest and best, and relate one or two

during the breeding season. incidents connected therewith.

L. agrestis uses a thread in a similar manner.

L. maximus has been observed to lower itself a PULMONOBRANCHIATA.

distance of three or four feet by a thread. * Physa hypnorum. -As before stated, I have bad

Megalomastoma suspensum, inhabiting the West the young of this species creeping up and down

Indies, derives its name from its habit of suspend permanent threads for eighteen or twenty days ing itself from the branches of trees by a thread. I together. In one case, I saw three Physæ and a Limnæa glabra upon a thread of the former at one time. Often, when two Physæ meet upon the same Jeffreys, "Brit. Con.," vol. i. pp. 136-7. thread, they fight as only mollusks of this genus

# Ibid., p. 135.
• Lovell Reeve, “British Land and Fresh-water Mollusks,"

p. 26.

* Alder and Hancock, "Monograph of the Nudibranchiate Mollusca."

§ Guilding, quoted by Woodward, “Manual of the Mol. lusca," p. 200.

PECTINIBRANCHIATA.

servers of the economy of these creatures to frame

a more perfect one, I shall be the more satisfied Bythinia tentaculata.-This snail suspends itself

with my attempt. by a thread, after floating, which is usually

I must ask the reader to bear in mind that the attached to the surface of the water.

time and opportunities at my command for obRissoa parva is well known to conchologists as a

serving their life and habits do not admit of my thread-spinner. Mr. J. G. Jeffreys thus pleasantly

coming to the conclusion, that, because I have never speaks of it:-"Lying on a rock, by the brink of a

seen a species spin a thread, therefore it does seaweed-covered pool left by the receding tide, it is

not do so. On the contrary, I believe that all the no less pleasant than curious to watch this active little creature go through its different exercises, - less; and, doubtless, in their native habitat

, when the

Limneidæ use this method of travelling, more or creeping, floating, and spinning."

eye of man is not present to pry into their secrets, Several other species of Rissoæ spin threads, also

these seemingly insignificant creatures perform Barleeia rubra, Eulima intermedia, Cerithium reticu

these their appointed acts, while we, most wishing latum, Cerithiopsis tubercularis, and Pleurotoma

to see, see them not :nebula. An account of their different modes of pro

Planorbis lineatus.t-Inhabiting streams; could cedure will be found in Mr. Jeffreys's work, under

not spin a thread in its native habitat. I have not their several headings.

succeeded in keeping it alive long. Litiopa, a genus of small mollusks living on

Planorbis nitidus,t P. nautileus,t P. albus,t P. floating seaweed far from land, are said to use a

glaber,t P.vorter, P. spirorbis, * P. contortus, Lin. mucous filament for the purpose of regaining their

næa truncatula.t-Of these species, some spend station, after having been swept off the weed.* If

their lives on vegetation near the surface of ponds this be correct, we have a water-breathing mollusk

or pools, and others inhabit shallow ponds or using its thread as a means of ascent, after having

ditches, which sometimes become dry in summer ; spun it downwards, a circumstance I have not

hence the necessity for using a thread does not myself seen. My observation teaches me that these

often occur. threads are not used by mollusks against the laws

Planorbis carinatus, * P. complanatus.* - Living of gravitation.

in the larger ponds and pools where the water is of With regard to the spinning of Neritina fluvia considerable depth, this capability of thread-spintilis. This species is an inhabitant of running

ning often serves them to good purpose. streams, and will not live long in confinement. Its

Physa hypnorum, * P. fontinalis,* Limnæa glabra.* structure renders it impossible for it to spin an

-Inhabiting deep ditches, ponds, or pools, and fond upward thread, as the nature of its habitat alike

of indulging in subaqueous excursions, the habit of precludes it, and as it could not float in running

spinning is essential to their mode of life. water, it could not therefore spin a downward thread,

Limnæa stagnalis,* L. palustris,* L. auricularia,t as obtains with other members of its order. While

L. peregra, I L. glutinosa, I Planorbis corneus.tmaking these observations, I do not discredit Mr.

When full grown, these species, being much larger Warington's statement, because, although the act of

and stronger than any of the foregoing, are able to floating is not a normal one with the creature, it

traverse more ground in a given time; hence they might have performed it as mollusks sometimes

do not feel the necessity of using a thread so dont when placed under circumstances which allow,

often as the smaller species.
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.

HEMIPTERA.
Having kept nearly every British species of the
Limnæidæ in confinement on purpose to observe

(Continued.) their babit of spinning, and not having seen some

HOMOPTERA.—THE FROG-HOPPER (Aphrophora species use this means of locomotion at all, others

spumaria). seldom, and some often; some when young but less

N last October number of SCIENCE-GOSSIP I often as they grow older, and others all their life

drew the attention of its' readers to the ovipotime, I have been led to advance a theory whereby

sitor saws and suctorial tubes of the Nettle-bug to account for this varied use of these threads. To

(Stenocephalus agilis), order Heteroptera, or this end I have drawn up the following table. While

ferent wings.” writing it, I am sensible of its imperfections; but if

From communications received, and numerous it only serves as a nucleus to stimulate other ob

applications for mounted slides of ovipositors, from

IN

“ dif

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

† For an account of this habit in Trochus occidentalis, a deep-sea species, see Jeffreys, “Brit. Con.," vol. iii. pp. 335-6.

Species I have seen spin a thread.
† Species I have kept, but not seen spin.
I Species seen to spin by others.

several contributors of this journal, I fancy some of tibia and tarsus being terminated with toothlittle interest has been excited to further investigac | shaped spines (fig. 45). tions of these organs; for that reason I feel induced The elytra when subjected to microscopical exto give a brief sketch of another order of Homo amination will be found very beautiful. The ground. ptera, section Trissieræ, genus Aphrophora, species work is seen to be made up of cellular tissue or Aphrophora spumaria.

Fig. 41. Head and Awl-like antenna of Froghopper.

*

oval spots, arranged in irregular transverse rows. These spots are of uniform dimensions,* consisting of a centre, surrounded by a white circle, each spot being divided from its fellow by a space nearly equal

Fig. 43. Suctorial tube, lancets, &c., seen from beneath,

X 120.

Fig. 45. Posterior leg and terminal claw.

This insect, the “Frog-hopper,” will probably be of more general interest, for, although so common, it is little known. Many persons no doubt have seen the frothy secretion upon the branches and leaves of shrubs during the summer months, and probably know that it is commonly designated cuckoo-spit, and this, perhaps, is the only idea of many concerning it. This secretion is effected on the larva leaving the egg. It covers itself with a froth, fixing its rostrum into the cellular tissue of the plant on which it is fixed; it draws up a sufficient quantity of sap to cover itself, pouring out a secretion from the organs placed at the terminal portion of the abdomen (this secretion is the spit); but a more careful observer will find, upon breaking it up, a small green larva, with yellow eyes; the insect thus protected passes through its larva of different stages until it arrives at maturity.

Upon subjecting the larva to the microscopic glass, the suctorial tube, lancets, &c., will be seen (fig. 43) composed of the labrum, which forms a jointed sheath for the slender trestle-like mandibles and maxillæ, and also a canal for the passage of the juices upon which the insects live.

In its mature state it is a dull, stone-bodied, inconspicuous insect with awl-like antennæ (fig. 44), from the last joint of which springs a trestle appendage. Posterior legs adapted for springing, the extremity

to its own diameter; and, when viewed as an opaque object, stand out in relief. The females are furnished with a singular and beautiful apparatus (fig. 46), by which they are enabled to form excavations or grooves in the twigs or leaves of plants, for the purpose of depositing their eggs (which are large and few in number). It is analogous to the instrument possessed by the Tenthredo or Saw. flies. On the under surface of the terminal segment of the abdomen, nearly at the extreme point, are seen a pair of valves or palpi, which form the sheath for the auger or boring instrument. Upon a casual view this auger appears like a denticulated arrowheaded spear, but on a more minute examination it is seen that what appears at first sight a single instrument, is made up of four distinct portions, two deeply indented blades set back to back, and a middle support, in which they slide: the remaining two have their outer edges smooth, but the inner are cut into the most regular minute serrations. These facts are probably known to many; still there may be a few to whom this brief sketch may be in

* This is more discernible when the elytra is mounted on balsam.

« AnteriorContinuar »