Elementary Zoology Part 12

These processes are the _l.a.b.i.al palpi_ and serve to protect the sucking proboscis. The proboscis itself is composed of the two greatly modified _maxillae_. Instead of being short, jaw-like and composed of several pieces as in the locust, in the b.u.t.terfly each maxilla is a slender, flexible half tube applied against its mate on the opposite side in such a way as to form a perfect tube long enough to reach into the nectaries of flowers when in use and capable of being compactly coiled up at other times. Cut across the proboscis and note the ca.n.a.l in the centre. Try to separate the two maxillae which compose it.

Make a drawing of the frontal aspect of the head with the eyes and appendages.

Compare the thorax with that of the beetle and that of the locust. The _prothorax_ is a freely movable narrow ring or collar. The _mesothorax_ and _metathorax_ are fused to form a large convex ma.s.s, of which fully five-sixths is mesothorax and only one-sixth metathorax. Try to distinguish the boundaries of the two segments. Note the three pairs of legs; the differences in size among them, and the differences between them and the legs of the locust and water-beetle. In one of the legs determine the _c.o.xa_, _trochanter_, _femur_, _tibia_ and _tarsal segments_. Note the differences between the wings of the b.u.t.terfly and those of the locust and beetle. Note that the wings are membranous, but are covered with many fine _scales_ (fig. 42), as is, indeed, the whole body. Rub off some of these scales on a gla.s.s slide and examine; note shape, little stem or pedicel of insertion, and longitudinal striations.

Examine under microscope a bit of wing from which some of the scales have been rubbed. How are the scales attached to the wing membranes?

How are the scales arranged? Note that the wing is colorless where the scales have been removed. All the colors and patterns of the wings of b.u.t.terflies are produced by the scales.

Make drawings of scales; of parts of denuded wings, and of bit of wing covered with scales.

Remove all or nearly all the scales from a wing and note the arrangement of the _veins_ (_venation_). Compare with venation in wings of locust.

Make drawing showing venation in the b.u.t.terfly's wings.

[Ill.u.s.tration: FIG. 42.--Bit of wing of Monarch b.u.t.terfly, _Anosia plexippus_, magnified to show the scales; some scales removed to show the insertion-pits and their regular arrangement. (From specimen.)]

The venation of insects' wings is much used in insect cla.s.sification, and the various veins have been given names. The names of the veins in the b.u.t.terfly's wings are given in fig. 43. When the veins in the wings of all the various groups of insects are studied, it is evident that the princ.i.p.al ones are the same in all insects, so that the costa, sub-costa, radius, media, cubitus and a.n.a.l veins of the b.u.t.terfly's wings can be compared with the corresponding veins in the wings of a beetle or wasp or fly. Noting the differences in the number and character of branching of these princ.i.p.al veins, and the number and disposition of the cross-veins which connect the longitudinal veins, the various kinds of insects can be to a large extent properly grouped or cla.s.sified. A detailed account of the wing-veins of insects is given in Comstock and Kellogg's "Elements of Insect Anatomy," chap. VII.

Of how many segments is the abdomen composed? The first or basal segment is depressed, while the others are more or less compressed. The _spiracles_ are, as in the locust, situated on the lateral aspects of the abdominal segments. What segments bear spiracles? The terminal segments of the abdomen differ in the two species. In the female the dorsal part of the (apparently) last segment is longer than the ventral part and is bent down over it forming a sort of hood over a s.p.a.ce enclosed partly by this hood, partly by a bluntly-pointed projection from the ventral surface, and partly by the lateral margins of the segment. In this chamber lies the opening from which the eggs issue. In the male there are several backward-projecting, h.o.r.n.y, thin processes.

[Ill.u.s.tration: FIG. 43.--Wings of monarch b.u.t.terfly, _Anosia plexippus_, to show venation; _c_, costal vein; _sc_, sub-costal vein; _r_, radial vein; _cu_, cubital vein; _a_, a.n.a.l veins. In addition most insects have a vein lying between the sub-costal and radial veins called the median vein.]

Make a drawing of the lateral aspect of the whole body.

=Life-history and habits.=--The tiny, conical, yellowish-green eggs of the monarch b.u.t.terfly are deposited on the under side of the leaves of milkweeds (_Asclepias_) and when examined under the microscope are seen to be very beautiful little objects finely ribbed with longitudinal and transverse striae. The eggs are laid in April and May (depending on the lat.i.tude and season) by females which have hibernated in the adult condition. From the eggs the minute, cylindrical, pale-green, black-headed larvae hatch in four or five days. As soon as hatched the larva devours the eggsh.e.l.l from which it has escaped and then feeds voraciously on the milkweed leaves. It grows rapidly, and in three or four days a blackish band or ring appears on each segment, and for the rest of its life it is very conspicuously colored with its black rings on a yellowish-green background. It molts three times, and in from twelve to twenty days is ready to pupate, or change to a chrysalis.

When ready to pupate the larva usually leaves the milkweed plant, and seeks some such protected place as the under side of a fence-rail or jutting rock. Here it attaches its posterior extremity by a small silken web to the rail or rock, and casting its larval skin appears as a beautiful pale-green chrysalis with ivory black and golden spots. It hangs motionless, and of course without taking food, for from a week to two weeks (according to season and temperature), when the pupal cuticle breaks and the great red-brown b.u.t.terfly (fig. 165) issues.

The b.u.t.terfly feeds (as is indicated by the structure of its mouth-parts) very differently from the larva; it sucks up by means of its long tubular proboscis the nectar of flowers, nor does it confine itself at all to the flowers of milkweeds. It is a fine flyer and a great traveller. Many thousands of these b.u.t.terflies often make long flights or migrations together. At other times tens of thousands of these b.u.t.terflies congregate in a certain limited area, clinging sometimes to the branches of a few trees in such numbers and so closely together as to give the tree a brown color. Such a "sembling"

of monarch b.u.t.terflies occurs every year near the Point Pinos lighthouse on the Bay of Monterey, California. The object of this a.s.sembling together is not understood. Both the larvae and adults of the monarch b.u.t.terfly are distasteful to birds, by their possession of an acrid body-fluid. The species is thus protected against the most dangerous enemies of b.u.t.terflies, a fact which chiefly accounts for the great abundance and wide distribution of the monarch (see p. 137).

For a full account of the life-history of the monarch b.u.t.terfly, see "Scudder's Life of a b.u.t.terfly."

LARVA OF MONARCH b.u.t.tERFLY (_Anosia plexippus_)

TECHNICAL NOTE.--For directions for finding and identifying the larvae of the monarch b.u.t.terfly see p. 171. If larvae (caterpillars) of _Anosia_ cannot be found, those of any other b.u.t.terfly or moth will do. Use naked, smooth kinds like cutworms, cabbage worms and the like, rather than hairy or spiny ones. Use large specimens.

Kill the caterpillar with ether or in a cyanide bottle.

=Structure= (fig. 44).--As we have learned from the study of the life-history of the locust, water-beetle and b.u.t.terfly, some insects are hatched from the egg in a condition resembling that of the parents in most structural characters. This is true of the locust. Other insects, as the beetle and b.u.t.terfly, are hatched in a form and condition apparently very different from that of the parents. The external appearance of a beetle or b.u.t.terfly larva differs much from that of the adult or imago of the same individual. It will be of interest to examine more particularly the structural condition of one of these larvae and to compare it with the structure of the adult.

[Ill.u.s.tration: FIG. 44.--Dissection of the silkworm, larva of the moth _Bombyx mori_.]

Is the body segmented? Is the body composed of _head_, _thorax_ and _abdomen_? Note the soft, flexible, weakly-chitinized condition of the _body-wall_. How many pairs of legs are there? Where are they situated? Is there any difference in the various legs? If so, what is the difference? Which of the legs of the larva correspond with the legs of the b.u.t.terfly? Why? The prothoracic segment and the abdominal segments 1 to 8 each bear a pair of _spiracles_ (small blackish spots on the sides). Are both _compound_ and _simple eyes_ present? How many eyes are there? Are there _antennae_? Dissect out the _mouth-parts_.

How do they differ from those of the b.u.t.terfly? Are they more like the mouth-parts of the b.u.t.terfly or more like those of the locust?

With fine sharp-pointed scissors make a shallow longitudinal incision along the whole length of the dorsal wall. In a freshly-killed specimen a drop of pale greenish blood will issue as the scissors'

point is first thrust through the skin. Put a droplet of this blood on a gla.s.s slide, cover with cover gla.s.s and examine with high power of the microscope. Note that the blood is a fluid containing numerous sub-circular or elliptical bodies, the _blood-corpuscles_. Note at least two kinds of corpuscles: most abundant a granular, circular kind, the true blood-corpuscles; and rarer, a larger, clear, usually elliptical or oval, but sometimes irregular and ambiform kind, generally spoken of as _fat-cells_.

Make a drawing of the corpuscles in the field of the microscope.

After making the dorsal longitudinal incision pin out the caterpillar in the dissecting-dish with dorsal aspect uppermost. When the edges of the skin are pinned back, the organs most conspicuous in the body-cavity will be the flocculent ma.s.ses of _adipose tissue_, the large, simple, tubular _alimentary ca.n.a.l_ usually dark or greenish because of the color of its contents, and the numerous silvery _tracheal tubes_. In those caterpillars which spin a silken coc.o.o.n, the _silk_ or _spinning-glands_ are usually long and prominent. They lie on either side of the anterior part of the alimentary ca.n.a.l, and open by a common duct on the labium.

Rising from behind the middle of the alimentary ca.n.a.l may be found the long, whitish, folded and twisted _Malpighian tubules_. By picking away the fat ma.s.ses, expose the full length of the alimentary ca.n.a.l. Note its great size (large diameter). Is it divided into distinct regions such as crop, proventriculus, stomach, intestine, etc.? How is it held in place? Trace the princ.i.p.al longitudinal tracheal trunks. Find, if you can, a pair of small compact bodies usually somewhat elongate, one lying on each side of the posterior part of the alimentary ca.n.a.l. These are the rudimentary reproductive organs.

Remove the alimentary ca.n.a.l by cutting it off at its posterior tip and also in the prothoracic segment. Work out now the _ventral nerve-cord_ and _ganglia_, and the _supra-sophageal_ (brain) and _infra-sophageal ganglia_ and the _commissures_ in the head.

In the body of the caterpillar we have found the same general disposition of organs as in the body of an adult insect, but several differences are nevertheless noticeable, viz., the presence of a large quant.i.ty of fatty tissue, the great size and simple character of the alimentary ca.n.a.l, and the undeveloped condition of the reproductive organs.

OTHER INSECTS

The cla.s.s Insecta includes those Arthropods which have one pair of antennae (sense appendages), three pairs of mouth-parts (oral appendages), and three pairs of legs (locomotory appendages). The insects, in further contradistinction to the crustaceans, are mostly land animals and breathe by means of tracheae or tracheal gills. They are the most familiar of land invertebrates, and, as already mentioned, include more species than are comprised in all the other groups of animals taken together. Beetles, moths and b.u.t.terflies, flies, wasps and bees, dragonflies and gra.s.shoppers are familiar members of the cla.s.s of insects, but spiders, mites, scorpions, centipeds and thousand-legged worms are not true insects and should not be so miscalled. These last belong to the branch Arthropoda but to other cla.s.ses than the cla.s.s Insecta. While insects are found living under most diverse conditions on land, that is, on the ground, in the leaves, fruits and stems of plants, in the trunks of trees or in dead wood, in the soil, in decaying animal or plant matter, as parasites on or in other animals, and in all fresh-water ponds and streams, they do not live in ocean water. A few species live habitually on the surface of the ocean, and a few other forms are found habitually on the water-drenched rocks and seaweeds between tide lines. The varied habits of insects, their economic relations with man, the beauty and grace of many of them, and the readiness with which they may be collected, reared and studied, renders them unusually fit animals for the special attention of beginning students of zoology.

[Ill.u.s.tration: FIG. 45.--A wingless insect; the American spring-tail.

_Lepidocyrtus america.n.u.s_, common in dwelling-houses. The short line at the right indicates the natural size. (From Marlatt.)]

=Body form and structure.=--The segments composing the body of an insect are grouped to form three body-regions, the head, thorax, and abdomen.

The head of an adult insect appears to be a single segment or body-ring, but in reality it is composed of several segments, probably seven, completely fused. The head bears the eyes, antennae and the mouth-parts.

The thorax is made up of three segments, each segment bearing a pair of legs. From the dorsal side of the hinder two thoracic segments arise the two pairs of wings which are the most striking structural features of insects. Not all insects are winged, (fig. 45), and of those which are a few have only one pair of wings, but the great majority of them have two pairs of well-developed wings (fig. 46), which give them, as compared with the other animals we have studied, a new and most effective means of locomotion. The great numbers of insects and their preponderance among living animals is undoubtedly largely due to the advantage derived from their power of flight. The hindmost part of the body, the abdomen, is composed of from seven to eleven segments, only the last one or two of which are ever provided with appendages. When such posterior abdominal appendages are present they form egg-laying or stinging or clasping organs.

[Ill.u.s.tration: FIG. 46.--A four-winged insect; a stone fly, _Perla_ sp., common about brooks. (From Jenkins and Kellogg.)]

The body-wall is usually firm and rigid, with thinner flexible places between the segments and body-parts for the sake of motion. The body-wall is composed of a cellular skin or hypoderm, and an outer non-cellular cuticle in which is deposited a h.o.r.n.y substance called chitin. This chitinous cuticle or exoskeleton serves as an armor or protective covering for the soft body within, and also as a point of attachment for the many muscles of the body.

Insects vary a great deal in regard to shape and appearance of the body, and certain of the external organs are greatly modified in different insects to adapt them to the varied conditions under which they live.

Especially interesting and important are the variations in the character of the mouth-parts and wings, the organs of food-getting and locomotion.

In our consideration later of some of the more important groups of insects the modification of these parts will be specially referred to.

Despite the great number of insects, however, and their varied habits of life, a strong uniformity of body-structure is noticeable, all of them holding pretty closely to the typical body-plan.

The most interesting feature of the internal anatomy of the insect body is the respiratory system. Insects breathe through tiny paired openings, called spiracles, in the sides of the abdominal (and sometimes the thoracic) segments (the number and disposition of the pairs of spiracles varying much in different insects). These spiracles are the external openings of an elaborate system of air-tubes or tracheae (fig. 47) which ramify throughout the whole body and carry air to all the organs and tissues. The blood has apparently nothing to do with respiration as it has in the vertebrate animals, where it carries oxygen to all the body tissues.

[Ill.u.s.tration: FIG. 47.--Piece of trachea (air-tube) from the larva of the giant-cranefly. (Photo-micrograph by Geo. O. Mitch.e.l.l.)]

[Ill.u.s.tration: FIG. 48.--The antenna of a carrion beetle, with the terminal three segments enlarged and flattened, and bearing many "smelling-pits", the antenna thus serving as an olfactory organ.

(Photo-micrograph by Geo. O. Mitch.e.l.l.)]

The other systems of organs are well developed and in many respects more complex and elaborate than those of any of the other invertebrates. The muscular system comprises a large number of distinct muscles, usually small and short, which are disposed so as to make very effective the various complex motions of antennae, mouth-parts, legs, wings, and egg-laying organs. The muscles appear to be very delicate, being almost colorless when fresh, but they have a high contractile power. The alimentary ca.n.a.l is divided into various special regions, as pharynx, sophagus, crop, fore stomach or gizzard, digesting stomach, and small and large intestine. From the ca.n.a.l just at the point of union of the digesting stomach (ventriculus) and the small intestine rise the so-called Malpighian tubules, which are excretory organs. They are long slender diverticula of the alimentary ca.n.a.l, and are typically six (three pairs) in number. The circulatory system is composed of a tubular vessel running longitudinally through the body in the median line just under the dorsal wall. It is composed of a series of chambers or segmental parts, which by a rhythmic contraction and expansion propel the blood anteriorly and into a short, narrow, unsegmented anterior portion of the vessel which may be called the aorta. There are no other arteries or veins, the blood simply pouring out of the anterior end of the dorsal vessel into the body-cavity. It bathes the body tissues, flowing usually in regular channels without walls. It re-enters the dorsal vessel through paired lateral openings in the chambers.

[Ill.u.s.tration: FIG. 49.--A section through the compound eye (in late pupal stage) of the blow-fly, _Calliphora romitoria_. In the centre is the brain, with optic lobe, and on the right-hand margin are the many ommatidia in longitudinal section. (Photo-micrograph by Geo. O.

Mitch.e.l.l.)]

The main or central nervous system consists of a large ganglion, the "brain," situated in the head above the sophagus, which sends nerves to the antennae and eyes, a ganglion in the head below the sophagus connected with the brain by a short commissure on each side of the sophagus, and sending nerves to the mouth-parts; and a ventral nerve-chain composed of a pair of longitudinal commissures lying close together and running from the head to the next to the last abdominal segment, which bears a series of segmentally disposed ganglia, each ganglion being composed of two ganglia more or less nearly completely fused. There is, in addition, a lesser system called the sympathetic system, which comprises a few small ganglia and certain nerves which run from them to the viscera. The function of the nervous system of insects reaches a very high development among the so-called "intelligent insects" and certain extraordinarily complex and interesting instincts are possessed by many forms. The social or communal habits of the ants, bees, and wasps and the habits connected with the deposition of the eggs and the care of the young exhibited by the digger wasps and other insects are of extreme specialization. The organs of special sense are highly specialized, the sense of smell (fig. 48) reaching in particular a high degree of perfection. One of the compound eyes (figs. 49 and 50) may contain as many as 30,000 distinct eye-elements or ommatidia, but the sight is probably in no insect very sharp or clear. Among insects there are organs of hearing of two princ.i.p.al kinds. In one kind the organ for taking up the sound-waves is a group of vibratile hairs usually situated on the antennae, as is the case with the mosquito; in the other kind, it is a stretched membrane or tympanum such as is found in the fore leg of a cricket or katydid or on the first abdominal segment of the locust (fig. 51).

[Ill.u.s.tration: FIG. 50.--Part of cornea, showing facets, of the compound eye of a horse-fly (_Therioplectes_ sp.). (Photo-micrograph by Geo. O. Mitch.e.l.l.)]

[Ill.u.s.tration: FIG. 51.--The auditory organ of a locust (_Melanoplus_ sp.). The large clear part in centre of the figure is the thin tympanum, with the auditory vesicle (small black pear-shaped spot) and auditory ganglion (at left of vesicle and connected with it by a nerve) on its inner surface. (Photo-micrograph by Geo. O. Mitch.e.l.l.)]

The s.e.xes are distinct in insects, and there is often a marked s.e.x dimorphism; in numerous species the males are winged while the females are wingless, and in a few cases this condition is reversed. Where there is a difference in size between male and female, the females are usually the larger. Fertilization of the egg takes place in the body of the female and, strangely, this fertilization is effected after the eggsh.e.l.l has been formed. In all insect eggs there is a minute opening in one pole of the eggsh.e.l.l called the micropyle through which the sperm-cells enter. In a few cases the young are born alive, but such a viviparous condition is exceptional. In a few species, too, young are produced parthenogenetically, that is, are produced from unfertilized eggs. And in the case of a few insect species male individuals are not known.

[Ill.u.s.tration: FIG. 52.--The young (at left) and adult (at right) of the bed-bug, _Acanthia lectularia_, a wingless insect with incomplete metamorphosis. (After Riley.)]