A. S. Packard

“ RELATIONS OF INSECTS TO OTHER ARTHROPODA The insects by general consent stand at the head of the Arthropoda. Their bodies are quite as much complicated or specialized, and indeed, when we consider the winged forms, more so, than any other class of the branch, and besides this they have wings, fitting them for an aërial life. It is with little doubt that to their power of flight, and thus of escaping the attacks of their creeping arthropod enemies, insects owe, so to speak, their success in life; i.e. their numerical superiority in individuals, species, and genera. ”

“ If we examine the tracheal gills of the smaller dragon-fly (Agrion) , or the May-flies, or Sialidæ, or Perlidæ, or Phryganeidæ, we see that they are developed in a very arbitrary way, either at the end of the abdomen, or on the sternum, or from the pleurum; moreover, in structure they invariably have but a single trachea, from which minute twigs branch out; [30] in the wings there are five or six main tracheæ, which give rise to the veins. Thus, in themselves, irrespective of their position, they are not the homologues of the gills. ”

“ The life of a winged insect, such as a butterfly, fly, or bee, may be divided into four stages: the embryo, or egg state, the larva, pupa, and imago,—the term metamorphosis being applied to the changes after birth, or post-embryonic stages of life. The transformations of the more specialized orders of insects involve wonderful changes of form, which are only paralleled in other types of animals by the metamorphoses of the echinoderms, of certain worms, and of the Crustacea, as well as by those of the frog. ”

“ It is nothing more than a regulator; than an organ for directing the blood in a determinate course in order that this may not wholly stagnate, or only be the plaything of a force acting in another way, as, for example, through that afforded by the body-cavity and the inner digestive canal. At regular intervals a portion of the blood is sucked through the same, and then by means of the anterior supply tube it is pushed onward into the head, whence it passes into the cavities of the tissues. ”

“ The best climbers are obviously those insects which live on trees and bushes, as, for example, longicorn beetles and grasshoppers. These may be accurately called the monkeys of the insect kind, even if their movements take place less gracefully, and indeed rather stiffly and woodenly. We already know what are the proper climbing organs; that is, the sharp easily movable claws on the foot. With the help of these claws certain insects, May-beetles for example, can hang upon one another like a chain; indeed, bees and ants in this manner bind themselves together into living garlands and bridges. ”

“ Indeed, if a luminous object is placed before a compound eye, it will illuminate a whole group of simple eyes or facets; moreover, the centre of this group will be clearer than the rest. Every movement of the luminous body will displace the centre of clearness; some of the facets not illuminated will first receive the light, and others will reënter into the shade; some nervous terminations will be excited anew, while those which were so formerly will cease to be. Hence the facetted eyes are not complete visual organs, but mainly organs of orientation. ”

“ More recently Mallock has given some optical reasons to show that Müller’s view is the true one. He concludes, and thus agrees with Plateau, that insects do not see well, at any rate as regards their power of defining distant objects, and their behavior certainly favors this view. It might be asked, What advantage, then, have insects with compound eyes over those with simple eyes? Mallock answers, that the advantage over simple-eyed animals lies in the fact that there is hardly any practical limit to the nearness of the objects they can examine. ”

“ Viallanes describes it as formed entirely of a very fine and close fibrillar web, like a thick hemispherical skull-cap, situated on the median line and united with the cerebral lobes. “It is like a central post towards which converge fibres passing from all points of the brain; being bound to the cerebral lobes, to the stalked bodies, to the optic ganglia, and to the olfactory lobes by distinct fibrous bundles.” ”

“ In the end the insects became, owing to this acquisition of wings, and afterwards to the establishment of a complicated metamorphosis, numerically the most successful type of life in existence, the number of species, extinct and living, mounting into the millions.
All aquatic insects are evidently the descendants of terrestrial forms, and the numberless contrivances and temporary larval organs, particularly of dipterous larvæ, are evidently adaptations to the needs of the insect during its aquatic life, and which are cast aside when the creature passes to a different medium. ”

“ Although Aristotle, as well as the natural philosophers of the Middle Ages, supposed that insects did not breathe, one can easily see that they do by holding a grasshopper or dragon-fly in one’s hand and observing the rhythmical rise and fall of the upper and lower walls of the abdomen, during which the air enters and passes out of the air-openings or spiracles on each side of the body. ”

“ It is evident that even the passive curve is not the imprint of the movement accomplished exclusively by the leg, for this latter, while struggling to reach its resting-place, is really involuntarily carried forward with the rest of the body. ”

“ Not only are the larvæ of each order of insects characteristic in form, so that the grub or larva of beetles is readily distinguished from those of other orders, or the maggot of flies from the apodous larva of wasps and bees, but within the limits of the larger orders there is great diversity of larval forms, showing that they are the result of adaptation to their surroundings. This is especially the case with the larvæ of the Coleoptera, Lepidoptera, Diptera, and Hymenoptera. ”

“ In general, however, in the development of the sexual outlets of insects, there are no characters which can be regarded as favorable to such a view. We must here accept the fact that the mode of development is secondary.
Mention should be specially made of the fact we owe to Heymons, that in the genital rudiment of Phyllodromia the genital cells and epithelial cells can be distinguished from each other from the very beginning. This fact does not favor the generally accepted view that the follicle-cells and egg-cells arise through a later differentiation from one and the same kind of cell. ”

“ Yet in the large, ordinary eggs of insects with an abundance of yolk there is no doubt, say Korschelt and Heider, that these numerous division-nuclei, which soon after the process of oviposition are scattered within the egg between the yolk-spheres, and are enveloped by a star-shaped protoplasmic layer, and which constitute the formative elements of the blastoderm,—there is no doubt but that they have practically arisen through indirect nuclear division from the first division-nucleus. ”

“ We must therefore, in insects, speak of a fore and a hinder endodermal rudiment. In proportion, now, as the ectodermal invaginations, which are destined to form the stomodæum and the proctodæum sink beneath the surface of the embryo, the cell-masses of which the two endodermal rudiments are composed are pushed farther in, and a separation between them and the mesoderm is thus effected. The two endodermal rudiments now form accumulations of cells which lie closely adjacent to the blind ends of the stomodeal and the proctodeal invaginations. ”

“ Development and mode of origin of the wings Embryonic development of the wings.—The wings of insects are essentially simple dorsal outgrowths of the integument, being evaginations of the hypodermis. They begin to form in the embryo before hatching, first appearing as folds, buds, or evaginations, of the hypodermis, which lie in pouches, called peripodal cavities. They are not visible externally until rather late in larval life, after the insect, such as a grasshopper, has moulted twice or more times; while in holometabolous insects they are not seen externally until the pupa state is attained. ”

“ In instructing small classes in the anatomy and metamorphoses of insects, it was strongly felt that the mere dissection and drawing of a few types, comprising some of our common insects, were by no means sufficient for broad, thorough work. Plainly enough the laboratory work is all important, being rigidly disciplinary in its methods, and affording the foundation for any farther work. ”

“ Not only by their structure and embryology, as well as their metamorphosis, do the myriopods and insects stand apart from the Arachnida and other arthropods, but it seems probable that they have had a different ancestry, the arthropods being apparently polyphyletic.
There are two animals which appear to connect the insects with the worms, and which indicate a separate line of descent from the worms independent of that of the other classes. These are the singular Peripatus, which serves as a connecting link between arthropods and worms, and Scolopendrella (Symphyla) . ”

“ The mid-intestine.—This section of the digestive canal, often, though erroneously, called the “chylific stomach” or ventriculus, differs not only in its embryonic history, but also in its structure and physiology from the fore and hind intestine of arthropods, and also presents no analogy to the stomach of the vertebrate animals. In insects it is a simple tube, not usually lined with chitin, since it is not formed by the invagination of the ectoderm, as are the fore and hind intestine, the absence of the chitinous intima promoting the absorption of soluble food. ”

“ The fertilization of the female takes place once for all a long time previous to oviposition; the semen in the receptaculum seminis passes out as the eggs slip down the egg-passage, and a spermatozoön gains entrance into the interior of the egg through the micropyle. In Œcanthus, according to Ayers, fecundation probably takes place while the egg is passing into the vagina, “since it is hardly possible that the male element could gain access to the follicles before the chorion is secreted.” ”

“ As soon as such expansions would be of even slight use to the insect as breathing organs, the question as to their permanency would be settled. Organs so useful both for flight and aëration of the blood would be still further developed, until they would become permanent structures, genuine wings. They would thus be readily transmitted, and being of more use in adult life during the season of reproduction, they would be still further developed, and thus those insects which could fly the best, i.e. which had the strongest wings, would be most successful in the struggle for existence. ”

“ Graber says that the heart of insects may be regarded not as an organ de novo, but only as the somewhat modified contractile dorsal vessel of the annelids, in which, however, the transverse arteries arising on each side became, with the gradual development of the tracheæ, superfluous and finally abortive. He describes it as a muscular tube composed of very delicate annular fibres, which within and without is covered by a relatively homogeneous, strong, elastic membrane. ”

“ Egg-cells, nutritive cells, and the cells of the follicle-epithelium (epithelium of the chambers of the ovarian tubes) are, says Lang, according to their origin, similar elements, like the egg and yolk-cells of the flat worms (Platodes) ; division of labor leads to their later differentiation. Only a few of the numerous egg-germs develop into eggs, the rest serving as envelopes and as food for these few. ”

“ Immature insects rarely possess compound eyes; they are only known to occur in the nymphs of Odonata and Ephemeridæ, and in the larvæ and pupa of Corethra.
Mode of vision by single eyes or ocelli.—In their simplest condition, the eyes of worms and other of the lower invertebrates, probably only enable those animals to distinguish light from darkness. The ocelli of spiders and of many insects, however, probably enable them, as Lubbock remarks, to see as our eyes do. The simple lens throws on the retina an image, which is perceived by the fine terminations of the optic nerve. ”

“ This process of transformation can only be understood by considering that of the embryonal germs of the organs, (1) only a part is destined for the use of the larva in growth, and for the performance of certain functions which exhaust themselves during larval life, so that it is no more capable of farther transformation, and finally becomes destroyed; while (2) a second part of the embryonal germs or rudiments persists first in an undeveloped condition, as imaginal buds, in order to undertake during the pupa stage the regeneration of the organs. ”

“ In the still more primitive, probably wingless, ancestors of insects there was a larger number of stigmata. Hatschek, in 1877, discovered a pair of tracheal invaginations in each of the three posterior head-segments of the embryo of a moth, with stigmatal openings in the 1st and 2d maxillary segments.
Thus early in embryonic life every segment of the body, except those bearing the eyes and the last abdominal, bore a pair of stigmata, so that the primitive insect had at least 15, and perhaps more, pairs of stigmata. ”

“ In the cockroach the vagina opens by a median vertical slit situated in the 8th sternite, into the genital pouch or bursa, upon the dorsal wall of which the orifice of the spermatheca is situated. In the sheep-tick the oviduct is enlarged to form the so-called uterus, which furnishes a milk-like secretion for the nourishment of the larva during its intra-uterine life.
In insects in general, the external opening of the vagina is simple, the chitinous structures (valves) at the opening being adapted to receive the male intromittent organ. ”

“ By the continuous growth of the primitive band around the yolk, after the resulting invagination and degeneration of the dorsal plate, the two blood-lacunæ unite together on the dorsal side into a single one (B, bs) . These constitute the first cavity of the heart. The vascular furrows (gr) come in contact with each other and grow together, and the wall of the heart is thus formed. Ayers states that in Œcanthus the heart is formed in the head region only after the yolk-sac has passed entirely within the body. ”

“ Certainly Scolopendrella is the only extant arthropod which, with the sole exception of the anteriorly situated genital opening, fulfils the conditions required of an ancestor of Thysanura, and through them of the winged insects. No one has been so bold as to suggest the derivation of insects from either diplopods or chilopods, while their origin from a form similar to Scolopendrella seems not improbable. Yet Uzel has very recently discovered that Campodea develops in some respects like Geophilus, the primitive band sinking in its middle into the yolk, with other features as in chilopods. ”

“ Development of the male germinal glands.—These rudiments arise in exactly the same manner as those of the female. Sexual differentiation takes place in the later embryonic stages. We then notice that in the male four masses of genital cells become surrounded by epithelial cells. These masses, which form the germs of the four testicular follicles of Phyllodromia, stand in intimate union with the rudiment of the vas deferens, and in the later stages move in connection with the latter, away from and behind the original genital rudiment. ”

“ The nervous system of insects consists of a double series or chain of ganglia connected by nervous cords or commissures. The first of these is the brain or supraœsophageal ganglion; it is situated in the upper part of the head, above the gullet or œsophagus, while the rest of the system, called the ventral cord, lies on the floor of the body, under the digestive canal.
A ganglion or nerve-centre consists of a mass of ganglion-cells, from each of which a process or fibre passes off, uniting with others to form a nerve ”

“ Certain insects also hum, and have what may perhaps be called a voice. The cockchafer, besides humming with the wings, produces a sound almost like a voice. In the large trachea, just behind each spiracle, is a chitinous process, which is thrown into vibrations by the air during respiration, and thus produces a humming noise. (Lubbock.) Such is also the case with flies, the mosquito, dragon-flies, and bees. In flies and dragon-flies the “voice” is caused by the air issuing from the thoracic spiracles; while in the humble-bee the abdominal spiracles are also musical. ”

“ It is apparent that the jointed or metameric structure of the bodies of insects and other arthropods is an inheritance from the segmented worms. In the worms the body is a continuous dermo-muscular tube, while in arthropods this tube is divided into regions, and the cuticle is thicker and more resistant. To go back to the incipient stages in the process of segmentation of the body, we conceive that the worms probably arose from a creeping gastrula-like form, the gastræa. ”

“ Weismann has given reasons for supposing that invaginated imaginal rudiments could not come into existence before the last larval moult in an insect whose life-history resembles that of Corethra or Chironomus. If the epidermis were invaginated in any stage before the ante-pupal one, the new cuticle, moulded closely upon the epidermis, would become invaginated also, and would appear at the next moult with projecting appendages like those of a pupa or imago. This is actually the way in which the wings are developed in some larval insects with incomplete metamorphosis. ”