Obrázky stránek




ITS ILAD. the worm, yet so slightly that they are liable to be detached by the slightest force, some of them falling off, sometimes, merely from the motions of the worm.

When these parasites issue from it the worm has become so weakened and exhausted that it ceases feeding and moving about, and in about three days afterwards all traces of its vitality have vanished. The multitude of minute hooks with which the soles of its pro-legs are furnished, however, continue to hold the dead worm to the stalk of the plant, with its head hanging downwards and its body shrunken and flaccid from the evaporation of its fluids, until some agitation of the plant by the winds or other violence detaches it and it falls to the ground.

In the meantime the parasites change to pupa and after remaining in the cocoons seven days, they come out from them in their perfect form. The flies are black with clear transparent wings, and legs of a bright tawny yellow color, the hue of bees-wax, with the hind feet and the tips of the hind shanks dusky. They belong to the order HYMENOPTERA and to that group of the Ichneumon-flies which in works of science have been termed Ichneumonides adsciti or the family BRACONIDE. Several of the species of this family present the singular character of having the eyes pubescent, numerous fine short erect hairs arising from their surface. These pertain to a particular genus which has received the name Microgaster, from two Greek words, equivalent to our English term "small-bellied.” It is to this genus that these parasites of the Tobacco-worm belong. And they were described by Mr. Say, in a posthumous paper which was published in the year 1835, in the Boston Journal of Natural History, vol. i, p. 262, under the name Microgaster congregata or the Congregated Microgaster, in allusion to their young being found together in such numbers upon a single


The TOBACCO-WORM PARASITE, Microgaster congregala, is of a coal black color and 0.14 long when living. After death it contracts in drying and is then scarcely 0.12 in length, and the male is a size smaller, not exceeding 0.10. Its head is spheroidal, or of a flattened globular form, with the antennæ inserted in the middle of the front side. The antennæ are coarse, thread-like, and longer than the body in the male, shorter in the female. They are composed of about seventeen joints so closely connected that their articulations are difficult to perceive. The joints gradually become slightly shorter and less thick as they approach the tips. The palpi and jaws are white. The eyes are distant from each other on the sides of the head, and in a strong light their surface is seen to be closely bearded over with minute short hairs. Between them on the crown the eyelets or ocelli appear as three small glassy dots placed at the corners of a triangle. The thoras is the broadest part of the body. It is egg-shaped, its surface minutely and closely punctured, and back of the middle it is crossed by a deep groove. The abdomen is oblong oval and about the same length as the thorax. It is smooth and shining, except the two first segments which are rough from obscure shallow punctures, with an elevated longitudinal line in the middle. On its underside the three first segments are pale yellow, with a dusky TOBACCO-WORM. PARASITE DESCRIBED. ITS WINGS. spot on the middle of each, that on the third segment being large, and as the sutures contract in drying these spots become united. At its tip the abdomen in the female is compressed and vertically truncated, with the sting forming a conspicuous projecting point at the lower end of the truncation. In the male the tip is rounded and without any projecting point, though when living it may sometimes be seen to protrude two styles or slender cylindrical processes pointed at their tips, and between these a thicker process from the apex of which a fine bristle is occasionally thrust out. The legs are bright tawny yellow, becoming more dull and pale in the dried specimen. The hind feet and tips of the hind shanks are smoky or blackish. The hind thighs are also blackish at their tips and frequently show a dusky line along their upper sides, extending nearly to the base The wings are hyaline, glassy and iridescent. The forward pair have the stigma appearing as a large, opake, triangular, brownish black spot on their outer side slightly beyond the middle. The rib or marginal vein is thick and brownish black, becoming paler brown towards its base. The basal portion of the wing is traversed by two pale longitudinal veins, which are parallel, the outer one straight, the inner one curved towards its base. The outer vein sends off a long and nearly straight branch obliquely outward and backward to the anterior end of the stigma, this branch bounding the discoidal and the first cubital cells on their fore sides. The discoidal cell is triangular, with the vein on its inner side brown and angularly bent at one third and again at two thirds of its length, giving off at each of these angles a short oblique veinlet, the first one of which is brown and the other colorless. The first cubital cell is of the same size with the discoidal and is irregularly six-sided, the anterior and the inner sides being quite short; and the veinlet bounding this cell posteriorly is thick and brownish black, the inner half of its length being oblique and the outer half transverse, ending in the inner angle of the stigma. Beyond this,

, traversing the apical third of the wing are three longitudinal veins, which are very slender and colorless. The middle one of these veins is abruptly thickened and blackish brown for a very short distance at its base, this thickened portion forming, with the oblique inner end of the veinlet last described, two of the sides of the small triangular cellule which is common in the wings of the insects of this genus and family, but the short veinlet which should complete the inclosure of this cellule on its hind side, is wholly wanting

Mr. Say is wholly silent respecting the interesting habits of this insect, merely remarking that he obtained eighty-four of the flies from the larva of a Sphinx in the month of June. As I have had the flies come from the cocoons in July and also in September, it is probable that they are abroad upon the wing during the whole summer season, actively searching for suitable worms to inoculate with their eggs. As will be seen from a statement in one of the following pages, this parasite does not appear to be limited to the tobacco-worm, but preys upon the larvæ of other species of Sphinx also. And some of our other species of Microgaster have the same habit of fastening their cocoons to the larvæ from which they respectively




PARASITE'S COCOONS MISTAKEN FOR EGGS. issue. It is not rare, therefore, to meet with a worm which is thus burthened and shackled; and they are justly regarded as great curiosities. Correspondents have frequently sent me examples of this kind; some of them supposing in the fullest confidence that the little cocoons adhering to the back of the worm were eggs which the worm had laid, thus demonstrating, as it was thought, that the statements made in these Reports were erroneous, that it is only in their perfect and never in their larva state that insects produce eggs. This is an error into which every one who is not acquainted with insects and their wonderful habits and transformations will be very apt to fall, the shape, color and size of these cocoons being so much like eggs which a large worm like this might be expected to generate. And it shows in a strong light how important it is that our population should be correctly informed and measurably intelligent in this science. For a person destroying one of these worms will be particularly careful to also destroy all these supposed eggs; deeming that in each one of them he in effect destroys another worm; instead of which he hereby protects and insures the upgrowth of another worm-thus doing the very thing which he is aiming to prevent !

Of the hundred flies which are bred from one of these Ichneumonized tobacco-worms, we may assume that fifty at least will on an average be females, to destroy fifty more worms.

We thus see what efficient agents these insects are in checking the increase of this moth, and what an important service they hereby render us. Indeed, when we recur to the fact that these parasites attain their growth in a space of time so very much shorter than does the tobacco-worm, whereby there is probably two generations of them to one of the latter, it will appear that the parasites issuing from a single Ichneumonized worm will suffice to destroy two thousand and five hundred other worms within the time that one brood of these worms is growing up to maturity. They would therefore speedily exterminate these worms from existence, were they permitted to go on multiplying themselves without any check. And they are so well secreted and protected that there would seem to be little risk of their being discovered and destroyed by any enemy. For during their larva state, when they are soft and tender and without feet or any other means of defence or escape, they are lodged within the body of the tobacco-worm where they are secure from harm ; and when they issue therefrom they immediately inclose themselves in tough paper-like cocoons, in which they lie hid until they have acquired wings wherewith to fly away from any danger which menaces them. Thus they would seem to be protected and safe from injury. Yet the artifice of enclosing themselves in cocoons fails to procure them immunity. Another minute insect has been created and endowed with the sagacity to discover them in the little pods in which they hide themselves, and there this creature metes out to them the same treatment which the tobacco-worm receives from them. Thus the tobacco-worm does not die unavenged. The lingering, miserable death which it has suffered, its enemies, as if by an act of retributive justice, are doomed to undergo in their turn.

TOBACCO-WORM, A DESTROYER OF THE PARASITE DISCOVERED. On one occasion, when I was contemplating one of the tobacco-worms which I met with covered over with parasitic cocoons, I noticed a very small fly wandering about among the cocoons. My first thought was that this fly was probably one of the Microgaster parasites which had just then come from some one of these cocoons ; but the query soon arose in my mind, whether it might not be an enemy, stinging the cocoons to destroy their inmates in the same manner they had destroyed the tobacco-worm. Its very small size did not enable the eye to discover whether it really was one of the Microgaster flies. I was so fortunate as to succeed in enclosing it in a small vial, and then upon examining it with a magnifier, I becamo assured it had not come from the cocoons, for I perceived it pertained to a different group of parasites from that to which the Microgaster genus belongs. But how could the highly interesting and important point be ascertained, whether it actually was a destroyer of the inmates of these cocoons ? With the hope of obtaining further light upon this subject a portion of the stalk of the plant with the tobacco-worm adhering to it was cut off and enclosed in a glass jar. On the fifth day thereafter, two Microgaster flies made their appearance in the jar, and the worm now being dead and beginning to become putrid, the cocoons were all removed from its surface and enclosed in a vial. It was feared that this slight violence to them had destroyed their inmates, as day after day now elapsed and no more flies came from them. But, three months later, in December, they being kept in a warm room, a dozen flies were discovered, wandering around in this vial ; and for some weeks after, others continued to come forth from the cocoons. And these proved to be identical with the single fly which had been captured among these cocoons so long a time before: It was therefore evident that that fly was the parent of these which were now issuing from the cocoons ; and so industrious had that little creature been, that it had punctured and dropped one of its eggs into all save two of the cocoons, which were more than a hundred in number; and these two, it is probable, would not have escaped, if the fly had not been interrupted and taken away from its work.

These destroyers of the insect which destroys the tobacco-worm are very small four-winged flies of a shining dark green color, with pale yellowish legs and white feet. They belong to the order HYMENOPTERA and the family CHALCIDIDÆ, and are closely related to the Hessian fly parasite, Semiotellus destructor, figured in my Seventh Report, plate 3, fig. 1, which figure will also serve to represent this insect in almost every particular. It pertains to the genus Pteromalus, a name derived from two Greek words, meaning bad wings, the wings in these insects being nearly destitute of ribs or veins. As they, by destroying the parasite of the tobacco-worm, cause that worm to be more numerous and hereby more injurious to the tobacco, and as they will often occur lurking about this plant in search of the cocoons upon which to bestow their eggs, they may not inappropriately be named the Tobacco Pteromalus. All the flies which came from the cocoons were females, from which the following description is drawn.

The Tobacco PTEROMALUS (Pteromalus Tabacum), is one-tenth of an inch

TOBACCO-WORY. PARASITE'S DESTROYER DESCRIBED. long to the end of its body, and is of a dark or bottle green color with a brassy reflection, and finely shagreened upon the head and thorax. The head is large and placed transversely, about three times as broad as it is long, convex in front and concave at its base. Viewed in front it is nearly circular, with a large oval eye slightly protruding upon each side, of a dull red color fading to brown after death. On the crown three ocelli or eyelets appear as glassy dots placed at the corners of a triangle. The jaws are yellow, their ends brown, with four minute teeth. The palpi or feelers are dull white. The antennæ are inserted in the middle of the face and when turned backward reach about half the length of the thorax. They become a little thicker towards their tips, and are of a brown color with the long basal joint dull pale yellow, and are clothed with a short incumbent beard. They are composed apparently of nine joints, the first joint being long and smooth, and forming an angle with the remaining joints. ' T'he second joint is the smallest of the series, being but little longer than thick and obconic in its form. The third joint is thrice as long and nearly thrice as thick as the preceding, and has the shape of a pear, the contracted portion of its base being formed of two rings or small joints which are rarely perceptible even in the live specimen when highly magnified, except these organs be put upon the stretch. The fourth and following joints are a third shorter than the foregoing, and are nearly equal and square in their outline, each successive joint very slightly increasing in thickness and diminishing in length. The last joint is about thrice as long as the one preceding it, of an oval or sub-ovate form, rounded at its base and bluntly pointed at its apex, and is probably composed as in the other species of this genus of three joints compactly united together. The thorax scarcely equals the head in width and is egg-shaped and thrice as long as wide. On each shoulder is a slightly impressed line extending obliquely backward and inward. The abdomen is a third shorter than the thorax, and in the live insect surpasses it in thickness, is egg-shaped and convex with its tip acute pointed. When dried it scarcely equals the thorax in thickness, and becomes strongly concave on the back and triangular when viewed from one side. It is smooth, polished and sparkling, of a green black color, the middle segments each with a broad purple black band visible in particular reflections of the light. Beneath it is black and at the tip shows some fine impressed longitudinal lines forming the edges of the groove in which the sting is inclosed. The legs are slender, pale wax yellow, with the feet and ends of the shanks dull white, the hips of the hind legs being stout and black, with their outer faces green blue and their tips pale yellow. The feet are five-jointed and dusky at their tips. The wings are transparent and reach slightly beyond the tip of the abdomen when at rest. The anterior ones are broad and evenly rounded at their ends, and have, near the outer margin, a thick brown rib or subcostal vein extending more than a third of their length and then uniting with the margin and terminating some distance forward of the tip, after sending off a short straight stigmal branch which is thickened at its end, with its apex notched. Towards the inner margin an exceedingly fine longitu

« PředchozíPokračovat »