Popular Science Monthly/Volume 39/June 1891/The Characteristics of Insects
|THE CHARACTERISTICS OF INSECTS.|
By LOUIS MONTILLOT.
INSECTS, arachnids or spiders, myriapods and crustaceans, are all included in the sub-branch of the arthropods or joint-limbs. Of the characteristics by which they are distinguished we mention here only the most salient. Insects have six legs, arachnids eight, and myriapods a more considerable number, but always short of ten thousand. The legs of crustaceans are variable in number; they have a carapace consisting of the external skeleton, which, being impregnated with carbonate and phosphate of lime, is always consistent and sometimes very hard.
The body of insects is divided into three distinct parts: the head, the thorax, and the abdomen. We may therefore define insects as articulated animals with six legs, having head, thorax, and abdomen distinct.
The life of the insect comprises four successive stages: those of the egg, the larva, the pupa, and the perfect insect. The rule, however, is subject to some interesting exceptions. The reproduction of insects usually takes place through sexual connection. The eggs are fertilized while passing through the oviduct of the female; but the females of certain bees, some of the butterflies, and several aphides, lay fertile eggs without the assistance of the male; this phenomenon is known as parthenogenesis. With some other species the females are viviparous.
The egg is composed of a firm shell, containing a limpid liquid that includes the germ of the embryo and the vitelline globules which are destined to nourish it. The eggs of insects are of the most diversified forms. The shells of many are adorned with remarkable figures. At the moment of hatching, the shell breaks, or rather opens like a hinged box-cover. A large number of the eggs look like seeds. Some are round, others cylindrical, conical, or hemispherical. Others represent solid forms, either flattened or terminating in a point. A curious study is furnished by the art with which the females deposit their eggs. Here we find single eggs; there we see them collected in considerable numbers within a parchment or silken protective envelope, which floats on the water, or is inserted in bark or attached to a stone. At other times we find a chaplet of symmetrical beads, arranged in closed rings around the branch of a tree. To deposit her eggs, the female bores with her ovipositor into the stems of plants, the tissues of animals, or the timbers of our houses. There are no old tree-trunks or cracked walls that may not serve her for a hatching place. The female takes care to place her eggs in conditions which will permit her progeny to find food for themselves from birth; but her solicitude usually ends at this point, while she leaves it to the rays of the sun to do the rest—a happy arrangement, for in most cases the parents die before the hatching. The embryo is developed rapidly under the influence of the ambient heat, and in a short time breaks the shell or springs the cover.
From the egg issues a being very little or not at all like its parents. Insects, indeed, undergo, before attaining full development, a series of transformations which are designated as metamorphoses. These metamorphoses are complete or incomplete; and there are even what we might call hypermetamorphoses, as in the case of the cantharides, the evolutional life of which, only recently well understood, is much more complicated than that of most other insects. The metamorphosis is said to be incomplete when the forms of the young insects, on coming out from the egg, are like those of the adult. Insects whose metamorphoses are complete come from the egg in the form of larvæ.
The growth of the insect all takes place during the larval state, and is very rapid. The superficial envelope soon becomes too small for the body it contains. So, at determinate periods of their existence, larvæ change their skin, or, to speak more accurately, burst the integument which envelops them, and shed it. This transformation constitutes the molting, which is repeated from three to eight times, according to the species. During these periods of transition, the larva, as if ill, loses its insatiable appetite, ceases to eat, and becomes stationary.
Insects of incomplete metamorphosis likewise acquire their full development through successive moltings. Each of the moltings is attended by a corresponding perfectionment of some part of the organism.
Larvæ of insects often have a horned head, with jaws that permit them to crush food. This conformation also occurs in the larvæ of insects which in the adult state have the mouth organized for suction. Some larvæ of Dipteræ, however, have the fore part of the body terminated by a pointed and retractile appendage; they might be spoken of as acephalous.
After the head come the rings, very like one another, and not exceeding a dozen in number. In these there are three varieties of structure: larvæ having only articulated legs (Fig. 2, upper); those having both articulated and membranous legs; and apodous larvae, or those having no legs (Fig. 2, lower).
In the first variety the articulated legs, which end in one or two claws, are attached to the three rings immediately following the head, each ring supporting a pair of legs. The same rings form, at a later stage, the thoracic casing of the perfect insect.
In the second variety the articulated legs are six in number. They are, besides, inserted as already described, but the fourth
Fig. 1. Eggs of Insects. 1. Eggs of Coleoptera (opened shell of Hydrophilus piceus). 2. Eggs of Moths (Bombyx neustria). 3. Eggs of Moths (Lasiocampa pini). 4. Eggs of Orthoptera (Acridides). 5. Eggs of Orthoptera (Blatta). 6. Eggs of Neuroptera (Hemeroba). 7. Egg of Diptera (Œstrides). 8. Eggs of Louse.
and following rings are furnished with fleshy protuberances, without articulations, which are designated as membranous legs.
Some larvæ bear silky tufts, others a sort of spines, and others fleshy appendages of curious forms. Many, like certain caterpillars, wear a brilliant livery, while others present a dull color that offers to the eye only false tones, passing from a dirty white to a maroon, with some parts—the head, for example—remaining darker.
Larvæ are to be found, we might say, everywhere and in everything—on the leaves and roots and in the interior of plants, under the ground, in putrefying matter, in the tissues of living animals, in cloths, and in water. Wherever they are found, these larvæ are busy, before everything else, in alleviating the hunger of the moment.
|Fig. 3.—Larva with Articulated Legs and with Membranous Legs.||Fig. 4.—Cases of Phryganides.|
They devour, and they gorge themselves, without taking any care to protect their existence against aggressors from without. But a more provident minority construct shelters for themselves—little movable houses which the animal carries with itself, and within which it withdraws in case of danger, like a turtle in its shell. The larva fixes itself to this refuge by means of its membranous legs, and moves without by its articulated legs. The materials necessary for the construction of the nest are gathered up in the element in which the animal lives. They are twigs, grains of sand, fragments of shells, collected in water by the Phryganeidæ, shreds taken from our cloths by the Tineidæ; earthy substances by the Chythra; and all are glued together by the secretions of the insect.
Whether in the open air or in water, or in the bottoms of underground chambers, larvæ, at a certain period of their evolution, undergo another change, and are transformed into nymphæ. Among the insects of incomplete metamorphosis, the nympha is but little different from the larva, and molts and is fed in the same way. Insects of complete metamorphosis act very differently: the larva becomes stationary and ceases to take food; the skin becomes like parchment, and slight movements of the abdominal rings, when they are touched, are the only signs of life. Yet, from the beginning of the transformation, on the seemingly half-dead being, appear distinctly, although masked by a thick veil, the antennæ, wings, and legs. These organs are symmetrically folded along the body, and the motionless nympha under the veil presents the forms of the perfect insect. It remains sometimes fastened to a wall or hanging from a branch by one of the silky threads which the larva has woven; in other cases it is enveloped in a firm shell or surrounded by a silken cocoon. Nymphæ thus protected remain in the open air, while others, less well guarded, find shelter from bad weather and the direct force of the sun often by being buried deep in the ground.
The insect secures its release from its nymphal envelope at the expense of violent muscular efforts. Sometimes the envelope splits in the back and gives passage in succession to the thorax, the head, the legs, and the wings; sometimes the hinged lid or operculum which closes the shell gives way under the pushing of the prisoner. At the moment of emerging, the young animal is not able to fly; its wings are crumpled and soft, but the circulation soon becomes more active, the wrinkles vanish, and the wings acquire sufficient consistency to permit the new being to take flight.
As we have already seen, the body of the adult insect may be divided into three principal and distinct parts—the head, the thorax, and the abdomen. The head (Fig. 6) bears the mouth, antennæ, and eyes; the three pieces that follow are. the three united rings that constitute the thorax. After the thorax comes the abdomen, the rings of which are movable and capable of gliding upon one another.
The head may be divided into several regions, which it is important to define well. They are, in fact, of great assistance in the description and determination of species. They are four in number: the front, including the space between the eyes; the vertex, or upper part of the head, behind the eyes; the cheeks, below and in front of the eyes; and the epistoma, sometimes called the hood, prolonging the front, and running down in front of the mouth. The head is generally articulated with the prothorax by a fine ligament, which leaves it more or less freedom of motion in all directions.
The mouth of insects is arranged to masticate, lick, or suck food. The buccal parts are of course fitted to their purpose, but the type to which all should be referred is the masticatory apparatus, which, by repeated applications to functions foreign to its
|Fig. 6.—Parts of which the Body of a Coleopter is composed. c, head; th', first ring; of the thorax (prothorax); th", second ring of the thorax (mesothorax); th'", third ring of the thorax (metathorax); a, abdomen.||Fig. 7.—Mouth of a Masticating Insect. ls, labrum; md, mandibles; mx, jaws and maxillary palpi; h, lower lip and labial palpi.|
primary destination, has been insensibly modified and at last transformed, according to circumstances, into a licking or sucking apparatus. The buccal organs of masticating insects move laterally, after the manner of scissors, while the jaws of vertebrates work up and down, like pincers.
When we examine the mouth of the masticating insect, we find in it parts in the order indicated in Fig. 7. First is an upper lip, or labrum, ls; second, a pair of upper jaws, or mandibles, md; third, a pair of lower jaws furnished with maxillary palpi, mx; and fourth, a pair of labial palpi supported by a lower lip, h, which is itself attached to the border of the buccal cavity forming the chin. The articulated mandibles on the cheeks are strong pincers, which sometimes attain a very considerable development. They are often dentated on their inner border.
The lower jaws are composed of several pieces. Their inner lobe is frequently furnished with a brush of silky threads, and sometimes ends in a little mobile tooth, such as may be remarked among the Cicindelæ, (Coleoptera). The outer lobe is often articulated in the same way as the palpi. The insect may then be regarded as having two pairs of maxillary palpi.
The maxillary palpi include from one to six pieces or joints. The labial palpi have from two to four. The intermediate piece, or tongue, is subject to great modifications. With the chin, it completes the buccal pieces.
In the licking and sucking insects the organs we have described are adapted to their new functions. Thus the tongue in bees reaches a great development; bugs, grasshoppers (Fig. 9), and lice have a long beak enveloping silky hairs, which form rudiments of jaws; in the butterflies we perceive a disproportionately
|Fig. 8.—Mouth of a Bee. b, tongue; c c, labial palpi; d, mandibles; e, jaws; h, inner lobe; o, upper lip, or labrum.|
|Fig. 9.—Beak of a Grasshopper.Fig. 10.—Trunk of a Butterfly.|
long proboscis; while in the Diptera the dispositions vary with the different groups. But in all these transformations the attentive observer will be able to perceive vestiges of the pieces comprising the mouth of the masticating insect which we have chosen as a type. These homologies were clearly established by De Savigny in 1816.
The eye and the head in vertebrates are movable. The visual rays consequently embrace a large horizon. The eye of the insect, on the contrary, is immovable, and solidly incased in the head; and the movements of the head itself are very limited. A great inferiority would result from this, had not Nature compensated for it by augmenting the circle of action of the eye itself. The insect's eye is formed by the union into a single mass of a considerable number of little eyes—sometimes exceeding twenty thousand (Fig. 11). Each of these minute organs, which are easily distinguished with a glass, comprises a hexagonal facet, representing the cornea; below this, a conical refracting mass represents the crystalline lens, and upon this abuts the nervous net emerging from the ganglion, which is itself in relation with the cerebral mass. The apparent part of the eyes is rounded, like a spherical cap, or rather like a portion of an ellipsoid. Sometimes the inner edge is indented like a bean, or like the kidneys of vertebrates. The eyes are then said to be reniform.
The organ of vision is sometimes made more complete by simple eyes, detached from the general group and permitting the insect to distinguish parts of space outside the field of the faceted eyes. These supplementary organs, called ocelli, usually three in number, are arranged triangularly between the reticulated eyes.
The antennæ are articulated appendages which insects bear on the head, near the eyes, sometimes forward of them, sometimes behind them, and which are often greatly developed.
The antennæ are generally regarded as organs of touch; there is hardly any doubt about the fact. Some naturalists make them the seat of the smell; others consider them connected with hearing; and some locate both senses in them. However it may be, they are formed of a series of articulations united to one another, the number and form of which furnish entomologists with good characteristics for classification.
The antennae are straight or bent. The basilar joint, which is in direct relation with the head, is called the scape; in bent antennae it is usually very large, and forms an obtuse angle with the next joint. The club, or terminal part of the antenna, is sometimes in the shape of an olive, and is composed of a variable number of joints. The whole number of joints between the scape and the club constitutes the funicle. Characteristic forms of antennæ are represented in Fig. 12.
The thorax comprises three rings more or less closely joined: the prothorax, mesothorax, and metathorax. The prothorax bears the first pair of legs; it is largely developed in the Coleoptera and Hemiptera, in which it appears as a horny buckler, and is freely articulated with the mesothorax. It is the corselet of the old authors, and is called the pronotum by the entomologists of to-day.
The mesothorax bears the second pair of legs and the first pair of wings, which are sometimes horny; we perceive on the upper part of the insect only a small triangular portion of it, which is hardly visible in some species, and is called the scutellum or escutcheon. All the rest of the dorsal part is covered by the wings.
The metathorax is closely united to the preceding ring, and frequently also to the first rings of the abdomen. It bears the second pair of wings, which are always membranous when they are not atrophied, and the third pair of legs. The ventral part of the thorax is called the sternum, and the lateral pieces are the epimera. The wings, usually four, are sometimes reduced to two, and may even disappear entirely, when the insect is said to be apterous. The two pairs of wings, when they exist, are unlike, as in the Coleoptera and bugs, or alike, as in the Hymenoptera and Neuroptera. When they are unlike, those of the first pair have a horny consistency. They form a protecting case for the wings of the second pair, and are called elytra; the wings of the second pair are membranous, and are supported only by a network of nerves, which forms a kind of framework for them. Of this character are the wings of the cockchafer and stag-beetle. In the elytrum we distinguish the base, or part adjacent to the prothorax; the shoulder, or fore-external part; the humeral callosity, a more or less pronounced lump near the shoulder; the suture, an inner part, by which the elytra at rest are in contact; the humeral angle, or external basilar angle (of the side of the shoulder); the scutellary angle, or inner basilar angle (of the side of the escutcheon); the summit or extremity of the elytra; and the sutural angle, formed by the line of the suture with the outer edge at the summit of the elytra.
The elytra are not always entirely horny. In the heteropterous Hemiptera, the elytra remain membranous for a considerable extent near the summit. When the two pairs of wings are alike, both are membranous, and are constituted on the same plan as the wings of the second pair in Coleoptera. Of this character are the wings of the bee, the hornet, dragon-fly, and butterfly. The wings of the last are furthermore covered with brilliantly colored scales. The wings of the Diptera (flies, gnats, etc.), with their finely reticulated nervation, present the same membranous appearance; but the second pair are wanting, the only representatives left of them being small appendages known as balancers. The legs are shaped for leaping (Fig. 13, A), for walking (B), or for swimming ( C ). Whatever may be their use, the general plan of their structure remains the same, and the modifications bear only upon one or the other of their constituent elements. Thus in the mole-cricket the legs of the first pair are adapted to digging the ground (D); those of the praying mantis (E F) are shaped like pincers; the lower leg, attached to the upper by a very supple joint, bends back upon it, as is shown in F, and forms with it a vise, the interior of which bristles with fine toothings suitable to hold captive any prey that may be taken. What it has been agreed to call the leg of an insect comprehends the coxa (a), the trochanter (b), the femur (c), the lower leg or tibia (d), and the tarsus (e). The coxa is short, and is articulated into a cavity of the epimerum called the cotyloid cavity. The trochanter, which follows it, assists the movements of the femur joint. The femur is a strong lever which, in the case of insects organized for leaping, is considerably developed in the hinder legs (Fig. 13). The leg has the form of an elongated trapeze, the major base of which is often provided with spurs, while the crest is covered with teeth or rigid hairs. The tarsus is composed of joints, not exceeding and not always reaching five in number. These short joints are of different forms. They are sometimes furnished with fine balls of silk, a kind of brushes which aid in standing—or with suckers answering the same purpose. The last joint of the tarsus, called the onychium, bears one or two nails. The joints of the tarsi are generally distributed in equal numbers upon all the legs of the insect; but there are sometimes fewer on the middle and after limbs than on the fore limbs.
The arrangements we have pointed out hold with the walking insects; with aquatic insects the rugosities of the joints are smoothed down, the nails are blunted, and the legs are transformed into ciliated paddles that permit the animal to move easily in the water.
The abdomen consists of a series of rings joined to one another by a fine membrane which gives them great mobility. It is by means of this disposition that the abdomens of females when distended with eggs attain such extraordinary proportions. The number of abdominal segments varies from six to nine. The last ones are sometimes transformed into accessories of the genital apparatus. The last horny arch of the abdomen is called the pygidium, as in the tail of the cockchafer.
The cutaneous envelope of insects is usually of a dull, ruddy, or pitchy color; sometimes clearer, sometimes of a metallic appearance; but that which constitutes their richest livery is the investment of their external skeleton. This investment is formed of silks, felted hairs, spines, or thin caducous scales, the overlaying of which composes most original designs. A moderate enlargement is sufficient to give some species the appearance of brilliant jewels; seen under the lens, the Curculio imperialis, a Brazilian beetle, appears like a real set of emeralds and diamonds. Nothing is more interesting than to observe the fine pubescence of our native weevils and the delicate scales of the coats of some individuals. All these ornaments are, however, so fragile that we only have to graze a butterfly's wing with the finger to scatter all the chatoyant dust, and to lay bare the transparent membrane which it covered.
Some Coleoptera, as the Larinus and the Polydrosus, become unrecognizable when an awkward hand has stripped them of their magnificent raiment; the emerald robe and the curious designs disappear, to give place to the dark coloring of the teguments. The circulatory apparatus of insects (Fig. 15) comprises a dorsal vessel (or heart), from which the ramifications start that distribute the life-giving fluid through the organization. The nervous network (Fig. 16) extends over the ventral face of the animal in the shape of two ganglionic cords which come together under the digestive tube to separate again into two branches. The latter embrace the œsophagus like a collar, and meet above it to inclose the cerebral mass.
As we have just seen, the nervous system is specially localized in the ventral region, while the seat of the circulatory apparatus is chiefly in the dorsal region. This is almost the exact contrary of what is remarked in vertebrates.
The digestive apparatus (Fig. 17) is situated between the circulatory and the nervous networks. The respiratory orifices, which give passage to the air, are distributed along the body. They are called stigmata. Upon them abut the tracheæ, tubes of an extreme tenuity, the interlacings of which branch out through the tissues.
The whole body of the insect is sensitive to touch, but the perception of sensations takes place chiefly by hairs in direct relation with the nervous system. These hairs are found more or less all over the creature, but particularly upon the antennæ and at the ends of the palpi.
The convenient arrangement of the eyes enables the insect to see all around itself without making any motion. The length of its view has not been precisely determined, but experiments seem to show that it is not great. The part played by the ocelli is also not clearly determined. In insects living in dark places, especially in those strangely shaped ones that live in caves, the organ of sight is atrophied, and is represented only by little swellings having the nervous filaments spread over their centers. These animals are not entirely insensible to the action of light; for they shrink from it and exhibit signs of being disagreeably affected when they are exposed to it.
The faculty of hearing is greatly developed among insects. The slightest noise disturbs them—yet the position of the organ of hearing has not been well defined. Maurice Girard assumes that its seat is in the antennaæ; and in the absence of a special organ, that it acts like a flexible rod, free at one end, and attached by the other end to an elastic membrane. M. Künckel agrees with Müller and Siebold that the organ of hearing is situated outside of the head.
The smell and the taste, on the other hand, belong entirely to the cephalic region. The taste is seated near the mouth; the smell is one of the appanages of the antennæ. This has been irrefutably demonstrated by M. Balbiani, who, taking a number of newly hatched male silkworm moths, and isolating them from contact with females, divided them into two lots, which he placed in different boxes. One of the lots was left undisturbed, the other was subjected to experiments. The pectinal antennæ of all the individuals in it were cut off at the roots. On bringing tables on which were females near the boxes containing undisturbed moths, the insects were observed, even at a distance of several yards, to beat their wings and become violently agitated. But when females were brought near the moths that had been deprived of their antennæ, they showed no signs of being affected; their wings remained flat and motionless. The strong exhalations from the females were imperceptible to them; the removal of their antennæ had deprived them of the power of smelling. Other naturalists, however, give the power of perceiving odors to the stigmata.
Locomotion is performed in insects, as in vertebrates, by means of levers moved by muscles; but in the vertebrates the lever is within and the motor muscles are without, while in insects the muscles are inside and the lever outside. We have already pointed out the adaptation of the legs to the habits and the abode of the animals. The hairs with which the tarsi are furnished have a part in permitting flies to move on polished objects and to assist the rapid course of hydrometers, or skippers, on the surface of water. The wings sometimes come into play in propulsion on water; they are then furnished with long cilia, forming broad oars. The mechanism of flight has been the object of important studies by Pettigrew and Marey. A wasp, the ends of whose wings have been gilded, presents the appearance as shown in Fig. 19. The dispositions of the elytra of the Coleoptera during flight are extremely various. Poujade has given some interesting drawings of them in the annals of the Entomological Society of France. In them the cockchafer, the Onthopagus, is shown simply raising its elytra, the Necrophorus lifting them in a plane perpendicular to that of the body, and another genus holding them closed as in a position of rest. Sometimes, also, in moving through the air, the middle legs are raised above the body.
Fig. 18.—Disposition of the Stigmata of a Silk-worm. S, stigmata of the first ring; S', stigmata of the fourth ring; S" to SVIII, stigmata situated on the fifth, sixth, and eleventh rings.
A considerable number of insects secrete products, some of which are useful, while others are injurious. First in order among useful secretions are silk and wax. Silk is furnished more or less abundantly by all the caterpillars and many other larvæ. The silkworms produce it in large quantities. Wax is produced by a number of Hymenoptera, which construct cells of it to hold honey. Aphides and cochineals secrete fatty matters, the white tufts of which form a kind of down on their bodies and on the plants they frequent. In other insects the secretions become a defensive armor. Thus the Hymenoptera drop poison in the wound made in animal tissues by their sting, which it causes to swell, An analogous stinging gives rise to the excrescences called galls, with which the leaves of trees are
Fig. 20.—Scaphinus repelling the Attack of a Carabus.
often covered. Many of the Coleoptera emit penetrating odors. The Cicindela smells of the rose, the Aromia moschata of musk; the anal glands of the carabicus produce butyric acid; while some of these Coleoptera eject by the anus a caustic liquid which, vaporizing suddenly, detonates with intensity, like an explosive; whence the Brachini have been called bombarders.
- Translated from the book L'Amateur d'Insects. Paris: J. B. Baillière et fils, 1890.
- Maurice Girard, Traité d'Entomologie, comprising the history of useful species and their productions, and of injurious species and the means of destroying them.
- Brehm, Les Insectes. French edition, by J. Künckel d'Herculais. Paris: J. Baillière et fils.
- See Leo Vignon, La Soie au point de vue scientifique et industriel. Paris, 1890 (Bibliothèque des Connaissanees utiles).