Part 21 (1/2)
How often has the enthusiastic young entomologist been subjected to sore disappointment by the parasitic habits of these _Ichneumonidae_! He has obtained some fine caterpillar, a great rarity, and by dint of much searching of his Westwood or his Stainton, feels quite certain that it is the larva of some much-prized b.u.t.terfly. He ascertains its leaf-food; which it eats promisingly; all goes on encouragingly. Surely it cannot be far from the pupa state now! When some morning he is horrified to behold, instead of the chrysalis, a host of filthy little grubs eating their way out of the skin of his beautiful caterpillar, or covering its remains with their tiny yellow coc.o.o.ns.
Some of these parasites are so minute that their young are hatched and reared in the _eggs_ of other insects. Bonnet found that the egg of a b.u.t.terfly, itself no bigger than the head of a minikin pin, was inhabited by several of the stranger grubs; for out of twenty such eggs, he says, ”a prodigious quant.i.ty” of the grubs were evolved.
A very interesting tribe of insects, so diverse from all other known forms as to const.i.tute an order among themselves, that of the _Strepsiptera_, pa.s.ses its youth in the bodies of certain wild bees. Mr Kirby's account of his first detection of one of these, though often quoted, is so interesting that I must cite it afresh. ”I had previously observed,” he remarks, ”upon bees something that I took to be a kind of mite (_Acarus_), which appeared to be immovably fixed just at the inosculations of the dorsal segments of the abdomen. At length, finding three or four upon an _Andraena nigroaenea_, I determined not to lose the opportunity of taking one off to examine and describe; but what was my astonishment when, upon my attempting to disengage it with a pin, I drew forth from the body of the bee a white fleshy larva, a quarter of an inch in length, the head of which I had mistaken for an acarus (_bee louse_)! After I had examined one specimen, I attempted to extract a second; and the reader may imagine how greatly my astonishment was increased, when, after I had drawn it out but a little way, I saw its skin burst, and a head as black as ink, with large staring eyes and antennae, consisting of two branches, break forth, and move itself quickly from side to side. It looked like a little imp of darkness just emerged from the infernal regions. My eagerness to set free from its confinement this extraordinary animal may be easily conjectured. Indeed, I was impatient to become better acquainted with so singular a creature.
When it was completely disengaged, and I had secured it from making its escape, I set myself to examine it as accurately as possible; and I found, after a careful inquiry, that I had got a nondescript, whose very cla.s.s seemed dubious.”
Mr Newman, in an essay of much value,[230] has shewn that the larvae of this tribe of insects are born alive, that they attach themselves to the abdomens of wild bees, nestling among the hair, and that they are thus introduced into the nest of the bee. Here it is somewhat uncertain how they are sustained at first, for at this time the bee-grubs are not hatched; probably they remain without food for some days, or devour a portion of the pollen and honey stored up. As soon, however, as the bee-grub is hatched, the Stylops-larva undergoes a metamorphosis, sheds its six legs, and becomes a footless maggot; it pierces the soft skin of the bee-grub, and feeds on its juices, till its maturity, as the Ichneumon on the body of the caterpillar.
When the perfect bee emerges in the following spring, it bears the full-grown Stylops, protruding from the rings of its abdomen. The latter is in pupa, all the organs being distinct and separate, but wrapped together, and inclosed in separate pellicles; very soon, it emerges, as described by Kirby, and escapes, leaving a great unsightly cavity in the body of the bee. This is the male: the female never escapes, but lays its eggs on the bee in which it has been reared, and then dies.
In the spring we frequently see among herbage a great uncouth beetle of a dark blue-black hue, with short wing-cases and long, heavy body, which discharges drops of yellow fluid when handled, and is therefore called the Oil-beetle (_Meloe proscarabaeus_). The early stages of this beetle have much affinity with those of the _Stylops_. The beetle lays a number of yellow eggs in a hole in the earth; these produce little active six-footed larvae, resembling lice, which crawl to the summit of dandelion and other flowers in the suns.h.i.+ne, and await the visit of a bee. On the arrival of one, the active grub immediately clings to its body, and is carried to the nest, not, however, to introduce itself parasitically into the body of the bee-grub, but to feed on the provision which the parent bee has stored up for its own young. Thus it becomes very fat, and grows to a size much larger than that of the full-grown bee-grub, having early dropped its six long clinging legs, which, having performed their proper function in catching hold of a bee, are no longer needed. It changes to a perfect beetle in autumn, lies in the bee's nest all the winter, and emerges in the spring.
The large jelly-like Medusae which in summer are seen floating around our coasts, driving themselves along by alternate contractions and expansions of their umbrella, are frequently infested by little creatures of widely different organisation, Crustaceans belonging to the genera _Hyperia and Metoecus_. On the beautiful _Chrysaora_ of the southern coast I have seen the _Metoecus medusarum_, a little shrimp about half-an-inch in length, with enormous l.u.s.trous green eyes, which takes up his residence in the cavities of the sub-umbrella,--dwelling in them as in so many s.p.a.cious and commodious apartments, of which he takes possession, evidently without asking leave of the landlord, or paying him even the compliment of a peppercorn rent. Here he snugly ensconces himself, and feels so much at home, that he is not afraid to leave his dwelling now and then, to take a swim in the free water, returning to his chamber after his exercise; and here he rears his numerous family, which, in the form of tiny white specks, very much unlike their parents in shape, stud the membranes of the jelly-fish.
But, what is stranger still, Mr M'Cready has recently discovered in the harbour of Charleston in North America, a _Medusa_ which is parasitic upon another _Medusa_. _Cunina octonaria_ does not swim freely in the water, but inhabits the cavity of the bell of _Turritopsis nutricula_.
”Not only does the latter furnish a shelter and dwelling-place for the larvae during their development; it also serves as their nurse, by allowing the parasites, whilst adhering by their tentacles, to draw nourishment out of its mouth by means of a large proboscis. In point of fact, the relation between them is of so unprecedented a nature, that the author may well be excused for having at first taken the impudent parasite for the gemmiparous progeny of the sheltering Medusa. The youngest state of this parasitic Medusa observed by the author formed a ciliated body of clavate form, adhering to the cavity of the bell by means of the slender stalk in which it terminated. The first change consists in the emission, from the thick end, of two slender flexible tentacles, and in the formation of a central cavity by liquefaction. At this stage of development, the author frequently observed gemmation taking place at the thicker end, sometimes frequently repeated.
Subsequently the number of tentacles becomes doubled. These bend together over the clavate extremity, and are then employed, instead of the thin end of the body, in adhering to the cavity of the sheltering Medusa. The thin extremity then acquires a mouth, and may be recognised as a stomachal peduncle, which is employed, as above indicated, in obtaining nourishment. The morphological nature of the proboscis becomes still more distinct when, after the lapse of some little time, an annular fold makes its appearance immediately under the tentacles, which is recognisable from its form, and from the formation in it of (eight) otolithic capsules, as the first indication of the future bell.
Simultaneously with the otolithic capsules, four rudimentary tentacles make their appearance between the four tentacles. The Medusa remains in this stage of development for a long time. The bell gradually becomes more freely developed, and at last, by the reduction and entire disappearance of the stomachal peduncle, becomes the most essential part of the Medusa, after it has left its previous dwelling-place in the bell of the _Turritopsis_. The bell nevertheless retains for some time its earlier lobed form and unequal tentacles.”[231]
More remarkable even than this a.s.sociation is the fact that certain true Fishes habitually reside in the stomachs of star-fishes. This circ.u.mstance, which had been observed in the Oriental Archipelago by MM.
Quoy and Gaimard, and by Dr Bleeker, has recently been confirmed by Dr Doleschall, who has written a very interesting Memoir on it.
This learned naturalist states that the fact of the connexion between the fish and the star-fish is well known to most of the fishermen in Amboyna, and that he was able to obtain a sufficiency of specimens for examination; but as the star-fishes (and with them the fishes) speedily died in confinement, he was unable to make continuous observations upon them in a living state. Of the results of his observations he gives the following summary:--
”The fish stands to the star-fish in a definite relation which cannot be the object of observation. Why the little fish should always seek the stomachal cavity of one and the same species of star-fish, and not that of various species, is a mystery. It is well known that Crustaceans of the genus _Pagurus_ inhabit the empty sh.e.l.ls of Mollusca; but we find on the sh.o.r.e the same species of _Pagurus_ in the sh.e.l.ls of the most various genera and species.
”I have never met with _Oxybeles gracilis_, on the contrary, in any other species of star-fish than _Culcita discoidea_. The fish was described by Bleeker under the above name in 'Natuurkundig Tijdschrift,'
vii., p. 162. The author proceeds to state that neither he nor any one else in Amboyna has ever captured the fish under other circ.u.mstances, or while swimming freely in the sea; but upon this Dr Bleeker remarks that many of his specimens of _Fierasfer Brandesii_, and all those of _Fierasfer (Oxybeles) gracilis_ and _F. lumbricoides_, were obtained by him along with other fishes, and were probably taken while swimming freely in the sea.
”Upon the habits of _Oxybeles gracilis_ the author goes on to say that it is certain that this animal pa.s.ses the greater part of its existence in the stomach of the star-fish, rarely shewing itself outside of this, and then probably at night. That it does come out occasionally, appears from the fact that in two cases the author observed the fish with a portion of its body outside the cavity of the star-fish, and in the act of creeping in.
”The same observations shewed that the fish, in returning to its concealment, pa.s.ses along the furrow of the lower surface of one of the arms leading to the mouth of the star-fish, which is wide enough, when the tentacles are retracted, to leave room for the pa.s.sage of the slender body of the _Oxybeles_. This fact likewise proves that the _Oxybeles_ does not get into the stomach of the _Culcita_ by accident.
”If a living _Culcita_ be cut in two, the fish is seen moving freely in the cavity of its body. If it be taken out, it immediately seeks the shade. If the two halves of the _Culcita_ (still alive) be placed in the water, the fish will soon be seen to draw towards them, in order to get into the cavity of the star-fish. When exposed to the light, it is uneasy, and its iris contracts excessively. The author never found two fishes in the same star-fish.
”In most of the fishes examined by him, the author found the stomach empty; it was full only in one. The contents of the stomach had the appearance of a lump of fat, and consisted of half-digested muscle.
Under the microscope, striated muscular fibres could be detected, and the author thinks that they belonged to the muscles of a fish. This circ.u.mstance proves that _Oxybeles_ does not feed upon the chyle of the star-fish, but that its nourishment is a.n.a.logous to that of other fishes. Whether it seizes upon the fishes taken by the star-fish for its own nourishment must be determined by further investigations.
”The author's observations establish--
”1. That _Oxybeles gracilis_ is not a true parasite.
”2. That it pa.s.ses the greater part of its life in the stomach of _Culcita discoidea_, as is also indicated by the unusually pale colour of the fish.
”3. That, however, it can come out, either to seek nourishment, or for the purpose of reproduction.
”4. That it returns to the mouth along the furrow on the ventral surface of the arms.