familiar example, eat almost every plant which comes in their way, and have been known to subsist, for a time, upon the dead remains of other insects. The great diversity of foliage eaten by the different varieties of the Datana ministra cannot, therefore, be admitted as proof of difference of species.

A much closer test of specific identity is the tolerance of a change of food-plant. If two insects very similar, but yet with such differences as to render their specific identity doubtful, be found feeding upon different kinds of plants, and if, upon transferring each of them to the foodplant of the other, they continue to feed and thrive, it is generally regarded as affording the strongest presumptive evidence that they are only varieties of one and the same species; and inversely, if they each refuse to eat the food of the other, that they are specifically distinct. This is no doubt, in the great majority of cases, a correct and sufficient test. Can we go farther and hold it to be an unexceptionable rule? If we assume it to be such, it will follow that some of the different forms of the Datana ministra are distinct species; for we have repeatedly tried to change the black walnut variety to the apple tree, and the walnut and sumach varieties each to the food of the other, and in every instance they have persistently refused to eat. But this test does not appear to us to be of such a nature as to make it infallable. If two broods of some indiscriminate feeder, such as the larvæ of the Spilosoma virginica above referred to, should be so situated that they would be compelled to feed, each upon some one species of plant, and if this restriction should be continued through many generations, it would seem very probable that their tastes might become so confirmed that each. would refuse to eat the food of the other, especially if the two plants were very unlike each other.

It will have been observed that the three trees to which we have above referred as the food-plants of the larvæ of Datana-the apple, the sumach, and the black walnut-belong to as many distinct families of plants, far removed from each other in their natural characters. But where their food-plants are botanically allied to each other, the larvæ can be transferred from one to the other without difficulty. I received, last summer, a number of half-grown larvæ which had been found upon the quince, and these were reared to maturity upon the leaves of the apple.

But let us proceed to describe some of the principal variations which occur in these insects, both in the larvæ and imago states, and see how far these variations seem to depend upon diversity of food-plants.

Without going into any extensive detail, we will take three of the most characteristic varieties, those found upon the apple, the sumach, and the black walnut, and point out the most striking differences. The larvæ described below are to be understood to have arrived at maturity.

The dissimilarity of the black walnut variety from the others, is very remarkable; so much so, that judging from the mature larvæ alone we should never suspect them to be the same, or even a closely allied species. But previously to the last moult, this variety is striped like the others, and it has precisely the same manners in feeding and moulting; and I have seen individuals, after the last moult, with short hairs as in the other varieties.

With respect to the coloration of the corresponding moths of these three varieties, a certain relation can be observed to that of their respective larvæ. The moths proceeding from the apple-feeding variety, are of a russet or reddish-brown color, varying considerably in shade in different individuals. These may be taken as the types of the Datana ministra proper. The moths from the sumach larvæ, with their broader and brighter yellow stripes, are of a pretty uniform buff-yellow color, with a conspicuous dusky spot a little below the upper extremity of the second transverse line, and a smaller dot opposite to it, between the first and second lines. This variety partially combines the characters of D. perspicua and D. major of Grote and Robinson, having the bright, buffyellow color of the former, but without its confluent first and second cross lines, and the discal spots and other characters of the latter.

The moths from the unmixed black caterpillars found upon the black walnut, are much darker than the others, being of a smoky-brown color of different shades, but sometimes approximating to the russet brown of the apple and the oak varieties. The space between the first and second cross lines is usually darker in this variety, than the rest of the wing.

Besides their diversities of color the varieties of Datana ministra differ also, but only in a slight degree, in the number and direction of the cross lines, the proportional width of the wings, and the depth of the crenulations or little rounded notches in the terminal margin of the upper wings. But Mr. Walsh has shown by the comparison of many specimens, (Proceedings of the Ent. Soc. of Philadelphia for 1865, page 194) that these variations pass into each other by insensible gradations, and therefore that they cannot be relied upon as specific characters.

Dr. Fitch has given an account of this insect in his second New York Report, and points out the principal varieties, but without referring them to their corresponding larvæ.

We have spoken above of the comparatively late season of the year when the Datana-moths issue from the pupa state in which they have passed the winter. This circumstance might suggest an interesting inquiry into the causes which produce the great diversity in the periods of insect life. Not only do animal, as well as vegetable organisms oo cupy every part of nature's domain, but they are also widely distributed as respects the seasons of the year. As we have our spring, summer, and fall flowers, so we have our early and late insects, destined to keep in check the earlier and later vegetation. One way in which this is brought about is an unexplained difference in the time required for the development of different species of insects in the pupa state. Of the many moths which pass the winter in this condition, some emerge in the month of May, a much larger number in June, whilst a few, like the moth of the fall web-worm, and the Datana ministra of which we have here been treating, often lie dormant till well into July. The ultimate or economical reason for this diversity is sufficiently obvious, but the actual or physiological causes which produce it, are unknown, and perhaps unknowable. All we can say is, that it is their nature to do so. Here, for instance, are two chrysalids, lying side by side, belonging it may be to closely allied species, and scarcely distinguishable from each other. Yet one shall feel the first touch of sping and come forth, whilst the other shall lie dormant and motionless, heedless alike of winter's cold and summer's heat, till at length the dial points to the predestined hour in the cycle of its existance. Then, without any known or visible cause, the vital forces begin to circulate, and the creature exhibits tokens of reviving animation, within the darkened chamber of its sepulture. Gradually it rouses itself from its lethargy, and throws off its swaddling cerements, and puts on its beautiful garments, and comes forth to the light and liberty of a more exalted state of existance.

We do not mean to say that these changes take place without the operation of natural causes, but that these agencies are of so subtile a nature, and of so delicate an adjustment, that they will probably forever lie beyond the reach of human investigation.

PRACTICAL TREATMENT.

This insect has never been known to increase to any very serious extent, and it cannot be regarded, at most, as more than a third-rate injurious insect. We have occasionally seen small apple trees nearly defoliated by them; but our own experience has been mostly with the black walnut variety. We have an ornamental row of these trees which we

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set out as seedlings twenty years ago. Scarcely a year has passed, since they attained a considerable size, that they have not been more or less disfigured by having some of their branches stripped of their foliage by these caterpillars. Some seasons they have been so numerous that we have thought it necessary to take measures for their destruction. They are eminently gregarious and therefore easily controlled. They feed side by side as closely together as they can stand, and when they are young a whole brood of them can be taken from the tree by removing a single compound leaf of the walnut. Later in the season, as we have above described, they come down, every time they moult, and mass themselves upon the trunk or large branches. As at this time they are bound together by web, they can be taken off en masse and consigned to to the flames.

They are, no doubt, kept largely in check by their natural enemies. We have reared from them two large species of Ichneumon flies, the Ophion mundum of Say, and another undetermined species.

I have often seen flocks of blackbirds alight upon the trees infested by these worms, but I was never near enough to determine whether they fed upon them. But I have seen that efficient destroyer of the hairy caterpillars, the American Cuckoo, in the act of devouring them. I observed that it always siezed the insect by one extremity, probably the head, and mashed it, by mumbling it between the tips of its mandibles for a time, before swallowing it. In the article upon Tussock-moth, in my first annual report, I described the manner in which I had seen the Cuckoo shave off the tufts of hair upon the larva of this moth, before swallowing it; but the larva of the Datana is but little hairy, especially previously to the last moult, and the only object that I can conceive the bird to have, in crushing the heads of these worms, is to destroy their vitality as nearly as possible. This is undoubtedly a wise precautionary measure, since the presence in the stomach of two or three dozen unharmed and squirming caterpillars could scarcely be compatible with that repose after a hearty meal, which is generally supposed to be conducive to digestion.

Explanation.—a, incipient gall on the under side of the leaf; b, corresponding bulge on the upper side; c, fully formed gall, showing the lips slightly separated, so as to permit the escape of the mature insects; d and e, incipient double galls, one being located on each side of the mid-rib; ƒ, the wingless female; g, the mature winged insect.

POPLAR-LEAF GALL-LOUSE.

(Pemphigus populicaulis, Fitch.)

Order of HOMOPTERA. Family of APHIDE.

The Cotton-wood (Populus monilifera, Aiton), though one of the least valuable of our shade trees, is nevertheless worthy to retain its place among our shade-trees on account of its rapid growth, and the ease and certainty with which it can be propagated, thus often becoming available where other trees have failed. For these reasons there is a considerable demand for it, in proof of which we may state that we have recently seen a statement of twelve thousand of these trees having been shipped, upon one order, to a Swedish colony in Nebraska. The health and growth of the Cotton-wood, and some other species of the poplar family, are sometimes seriously impaired by different kinds of excrescences on the twigs and leaves, constituting what are technically known as galls, and formed by as many different species of the gall-making aphides. One of these is a large corrugated gall, formed on the ends of the twigs, often near the tops of the trees, turning black when mature, and adhering to the tree through the winter, and making unsightly ex