Elementary Zoology Part 11

Although the head is apparently a single segment, it is really composed of six or seven body segments greatly modified and firmly fused together. Note that it bears a pair of large _compound eyes_ and three much smaller _simple eyes_ or _ocelli_.

TECHNICAL NOTE.--Strip off a bit of the outer covering of a compound eye, mount on a gla.s.s slide and examine under the microscope.

Note that, as in the crayfish, each compound eye is composed externally of many small hexagonal _facets_, the outer covering, the _cornea_, being simply the cuticular covering of the body, in this place transparent and divided into small facets. Besides the eyes, the head bears also several movable appendages, namely the _antennae_, and the _mouth-parts_. Note the number, place of insertion, and segmented character of the antennae. These antennae are sense-organs and are used for feeling, smelling, and, in some insects, for hearing. Note that the mouth-parts consist of an upper, broad, flap-like piece, the[10]_labrum_; of a pair of brown, strongly chitinized, toothed jaws or _mandibles_; of a second pair of jaw-like structures, the _maxillae_, each of which is composed of several parts; and of an under, freely-movable flap, the _labium_, also composed of several pieces. Each maxilla bears a slender feeler or _palpus_ composed of five segments. The labium bears a pair of similar palpi, which are, however, only three-segmented. The mandibles and maxillae, which are the insect jaws, move laterally, not vertically as with most animals.

Make drawings of the lateral aspect of the head; of a bit of the cornea; of the dissected out mouth-parts.

Of the three segments of the thoracic region of the body, the most anterior one is called the _prothorax_. It is freely movable and has a large hood or saddle-shaped piece, the _p.r.o.notum_, on its dorsal aspect, and a blunt-pointed tubercle on the ventral aspect. The foremost pair of legs is attached to the prothorax. The next segment is the _mesothorax_, which is immovably fused to the next thoracic segment. What appendages does it bear? The third segment is the _metathorax_, which besides being fused with the mesothorax in front, is similarly fused with the foremost abdominal segment behind. What appendages does the metathorax bear?

Examine one of the fore legs and note that it is composed of a series of unequal parts or segments. The segment nearest the body is sub-globular and is called the _c.o.xa_; the second segment is smaller than the c.o.xa and is called the _trochanter_; the third, known as the _femur_, is the largest of all; the fourth, _tibia_, is long and slender; and the next three, the last of which is the terminal one and bears a pair of claws and between them a little pad, the _pulvillus_, are called the _tarsal segments_. Most insects have five tarsal segments. Note the great size of the hindmost or leaping legs.

Determine the segments of the middle and hindmost legs. Make a drawing of a fore leg.

Examine the wings. In what ways do the front wings differ from the hind wings? The front wings are known as the wing covers or _tegmina_.

Note how the hind wings fold up like a fan, and are covered and protected by the wing covers. Draw the wings.

The abdomen is composed of a number of segments most of which resemble each other. The first segment (immediately behind the metathorax) has its dorsal and ventral parts widely separated by the cavities for the insertion of the hindmost legs. The ventral part of this segment is dovetailed into the ventral part of the metathorax and appears to be part of it. In the dorsal part of this segment there is on each side a spot where the cuticle is only a thin membrane. At these places are the auditory organs or ears of the locust. The thin membranes are the _tympana_. Only the various kinds of locusts and those insects closely related to them have ears of this kind. Most other insects are believed to have the sense of hearing situated in the antennae.

The abdominal segments from second to eighth are ring-like in form and are without appendages. There is on the side of each of these segments near its front margin a tiny opening or pore called a _spiracle_.

These spiracles are the breathing pores of the locust, which does not take in air through its mouth or any other opening in the head. There is a spiracle near each ear in the first abdominal segment, and one on each side of the mesothorax near the insertion of the middle legs.

The terminal segments of the abdomen are provided with certain processes which are different in male and female. The female has at the tip of its abdomen two pairs of strong, curved pointed pieces which compose the _ovipositor_, or egg-laying organ. The opening of the oviduct lies between the pieces. The male has a swollen rounded abdominal tip, with three short inconspicuous pieces on the dorsal surface.

Make a drawing of the lateral aspect of the abdomen of a female locust; also, of a male.

For a more detailed account of the external anatomy of a locust see Comstock and Kellogg's "Elements of Insect Anatomy," chap. II.

The external structure of the gra.s.shopper should be carefully compared with that of the crayfish; pay special attention to the mouth-parts and legs.

The teacher should point out the h.o.m.ologies and modifications.

=Life-history and habits.=--The eggs of the locust are laid in the autumn in the ground in bare dry places, as roadsides, closely-grazed pastures, etc. The female thrusts her strong ovipositor into the soil, and by opening and shutting it, thus boring, pushes in the abdomen for about two thirds its length. The eggs, about one hundred, are then deposited in a capsule or pod. The young locusts hatch in the following spring. When just hatched they resemble the parent locust in general appearance and structure except that they lack wings, and are of course very small. The young locusts are gregarious, congregating in warm and sunny places. They feed on green plants and travel about by walking and hopping. At night they try to find shelter under rubbish in the fields. They feed voraciously and grow rapidly, reaching maturity in about two months. During this post-embryonic development and growth they molt (shed the chitinous exoskeleton) five times. After the first molt indications of the wings appear in the shape of small backward and downward prolongations of the posterior margins of the dorsum of the mesothorax and metathorax. With each succeeding molt these wing-pads, or developing wings, are larger and more wing-like, until after the last molting they appear fully developed. With each molting, too, there is a marked increase in size of the locust, the average length of the body just before the first moult being 4.3 mm., before the second 6.8 mm., before the third 9 mm., before the fourth 14 mm., before the fifth 17 mm., and after the fifth (the full-grown stage) about 26 mm.

The molting is an interesting process, and can be readily observed.

The young locust ready for its last molt crawls up some post, weed, gra.s.s stalk, or other object, and clutches this object securely with the hind feet. The head is generally downward. The locust remains motionless in this position for several hours, when the skin suddenly splits along the back from the middle of the head to the base of the abdomen. By steady swelling and contracting and slight wriggling, lasting for half an hour to three-fourths of an hour, the old skin is completely shed, and the wings spread out. In an hour the wings are dry and the new chitinized exoskeleton firm enough for flying, or crawling about, and in another hour the locust begins to eat.

The red-legged locust does considerable damage to cultivated crops, but its injuries are insignificant compared with the tremendous losses occasioned by a near relative, the Rocky Mountain Locust (_Melanoplus spretus_). This locust has its breeding-grounds on the high plateaus of the Rocky Mountain region, but it sometimes migrates in countless numbers southeast over the plains and into the great grain-fields of the Mississippi valley. Such migrations occurred in 1866, 1867, 1874 (in this year eighteen hundred and forty two families in Kansas were reduced to dest.i.tution by the utter wiping out of their crops by the locusts) and 1876. With the settling-up of the regions in which the Rocky Mountain locust breeds, there seems to have come a change of conditions, so that no great migrations have occurred since 1876.

THE GREAT WATER-SCAVENGER BEETLE (_Hydrophilus_ sp.)

TECHNICAL NOTE.--The great water-scavenger beetles are large, black, elliptical insects common in quiet pools where they may be found swimming through the water, or crawling among the plants growing on the bottom. They are an inch and a half long and are readily distinguishable from all other water insects except the predaceous diving beetles (_Dyticus_). The antennae of _Hydrophilus_, however, are thickened (clavate) at the tip, while those of _Dyticus_ are thread-like for their whole length. The beetles may be readily collected with a water-net, and kept alive in gla.s.s jars or aquaria in water containing decaying vegetation.

=External structure= (fig. 39).--Is the body of the water-beetle composed of segments? Can you make out three body-regions, _head_, _thorax_ and _abdomen_? As in the locust the _metathorax_ is fused with the first abdominal segment and with the _mesothorax_, while the _prothorax_ is freely movable, and is covered above by a strong shield. The chitin armor of the whole body is specially heavy and strong, affording a great protection to the insect.

[Ill.u.s.tration: FIG. 39.--Ventral aspect of male great water-scavenger beetle, _Hydrophilus_ sp.]

On the flattened head note the _compound eyes_ and the peculiarly-shaped nine-segmented _antennae_. Are there any _ocelli_? Dissect out the mouth-parts. The beetle's mouth is fitted for biting, the mouth-parts being in general character like those of the locust, with distinct flap-like _labrum_, dentate _mandibles_, jaw-like _maxillae_ with long, slender, four-segmented _palpi_ and lip-like _labium_ with three-segmented _palpi_. Make drawings of the antennae and mouth-parts.

Note the character of the thoracic segments. Examine the wings and legs. The fore wings are modified into strong h.o.r.n.y sheaths, or _elytra_, which completely cover and protect the folded hind wings.

The hind wings are large and membranous. How are they folded? Note the adaptation of the middle and hind legs for swimming. Determine the various segments of the legs, i.e. _c.o.xa_, _trochanter_, _femur_, _tibia and tarsus_. Note the long longitudinal median keel on the ventral aspect of the thorax.

The abdomen articulates with the metathorax by the full width of the broad first abdominal segment. It is composed of a series of segments without appendages, of about equal length but decreasing in width from in front backwards. Of how many segments does the abdomen seem to be composed when viewed from the ventral aspect? From the dorsal?

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

TECHNICAL NOTE.--After examining the abdomen thus far, remove it from the rest of the body, and boil it in dilute pota.s.sium hydrate (KOH) in a test-tube. This will soften and partially bleach the body wall.

Examine the softened specimen, and note that at least two additional segments are to be found retracted or telescoped into the apparently last segment. The character of these terminal abdominal segments differs in male and female individuals, and specimens of both s.e.xes should be examined. (The males can be distinguished from the females by the peculiar pad-like expansion of the last tarsal segment of the fore legs.) Pull out the retracted segments, and note that they are unevenly chitinized, parts of their surface being simply membranous. Projecting backwards are several long-pointed processes. The female has but one retracted segment. Though the females of many insects possess more or less elaborately developed egg-laying organs, this is not the case with the beetles. Look for _spiracles_ near the lateral margins of the dorsal surface of the abdomen. How many pairs are present?

=Internal structure= (fig. 40).--TECHNICAL NOTE.--If fresh specimens are to be had, kill by dropping into the cyanide bottle (see p. 463). Specimens preserved in a 5% solution of chloral hydrate may be used if necessary. When putting specimens into this solution a small slit should be cut through the body wall to allow the preservative to enter the body cavity. When ready to dissect a specimen cut off the elytra and wings close to the base, and carefully remove all of the dorsal wall of the abdomen and thorax and the median portion of the dorsal wall of the head. Pin out, ventral side down, under water in a dissecting-dish.

[Ill.u.s.tration: FIG. 40.--Dissection of female great water-scavenger beetle, _Hydrophilus_ sp., the heart and tracheae being cut away.]

Note in the median dorsal line of the abdomen a pale transparent longitudinal vessel, the _heart_ or _dorsal vessel_. Note on each side of it six prominent triangles or "Vs" with apex of each directed laterally, the posterior three smaller than the anterior three of each side. These triangles are formed by respiratory tubes or _tracheae_.

From each spiracle or breathing-pore there extends into the body a respiratory tube or trachea. These lateral tracheae join a main longitudinal trachea on each side, from which are given off branches, which in turn repeatedly subdivide, until all parts of the body are ramified by tracheae, large and small, bringing air to all the tissues.

The oxygen is taken up from this air, and carbonic-acid gas is given up to it, when it pa.s.ses out of the body again through the spiracles.

Thus in the insects oxygen and carbonic-acid gas are not carried by the blood but by special air-tubes. The respiratory system of insects is very different from that of other animals.

Mount a bit of trachea in glycerine on a gla.s.s slide and examine under the microscope. Note the fine spiral line (looking like transverse annular striations) which is a thickening of the chitinous inner wall of the tube and which by its elasticity keeps the tracheal tubes open.

The heart, already noted, is composed of a longitudinal series of very thin-walled chambers, each with a pair of lateral openings into the body-cavity and with terminal openings into the adjacent chambers. The blood, which is colorless or greenish or yellowish, is sent forward through the successive heart chambers by regular contractions until it finally pours from the most anterior chamber freely into the body-cavity. Here it bathes the body-tissues, flowing perhaps in regular paths, giving up food to the tissues and taking up food from the alimentary ca.n.a.l, until it finds its way through the lateral openings into the heart chamber again. There are no arteries or veins.

Note the large ma.s.s of _muscles_ in the metathorax. Note, by attempting to remove it, that the anterior part of the muscle ma.s.s is attached to a chitinous part.i.tion-wall between the meso- and meta-thorax. Remove this part.i.tion-wall (and one between the metathorax and abdomen) and note that certain muscles run deeply down into the body. By pulling on the bits of chitin to which the muscles are attached, the muscles (if they have not been cut) can be stretched to the length of three-quarters of an inch. When released they will contract. (This stretching and contracting takes place only in fresh specimens.) What are these large and numerous muscles of the thorax for?

Remove the thin membrane stretching over the abdomen and in which the heart and tracheal "Vs" lie, and note immediately underneath it the large coiled _intestine_ with a knot of greenish yellow threads in the centre. Carefully uncoil and pin out the intestine, cutting away the tying tracheae, but being careful not to cut other structures. Work out the full length of the alimentary ca.n.a.l, noting the _sophagus_, the widened _crop_ behind it, and the long intestine. From the intestine arise several greenish yellow threads, the _Malpighian tubules_. These are the excretory organs of the insect. What is the total length of the alimentary ca.n.a.l?

The reproductive organs, consisting of a pair of glands (egg-glands or sperm-glands) with a pair of tubes which unite before reaching the body-wall and have a common external opening, may now be seen. These should be removed, thus exposing the _ventral nerve-chain_ in the abdomen. To expose the chain in the thorax it will be necessary to pick away carefully the muscles. As in the crayfish, the central nervous system in the beetle consists of a ventral nerve-chain, a _brain_ or _supra-sophageal ganglion_ and a pair of _circ.u.m-sophageal commissures_ connecting the brain and the foremost ganglion (_infra-sophageal_) in the ventral chain. There are, in the ventral chain, four ganglia in the thorax and four in the abdomen. The large nerves running from the brain to the compound eyes and to the antennae can be traced.

Make a drawing showing the nervous system.

=Life-history and habits.=--The eggs, usually about one hundred, are deposited in a silken sac or case which is spun by the female, and either floats freely or is attached to the under sides of the leaves of aquatic plants. This egg-case is not wholly filled with eggs but has a considerable air-chamber in it, causing it to float. It is oval in shape, and has a peculiar curved horn-like projection at the upper end. In sixteen or eighteen days the young water-scavenger beetles hatch as elongate, wingless, active larvae, provided with three pairs of legs and strong jaws. They remain for a short time after hatching in the egg-case, feeding on each other! After they issue from the case they feed on flies or other insects which fall into the water, and on snails. They breathe through a pair of spiracles situated at the posterior tip of the abdomen, coming to the surface and thrusting this tip up so that the spiracles are out of water. They grow rapidly, molting three times before becoming full grown. They attain a length of nearly three inches. When full grown they leave the water, crawling out on the damp sh.o.r.e of the pond or stream, and burrow into the soil for a few inches. Here they molt again, or pupate as it is called, changing to a non-feeding, quiescent stage called the pupal stage. The pupa is the stage in which the great changes from wingless, crawling and swimming, short-legged, long, slender-bodied larva to winged, swimming and flying, long-legged, compact, broad-bodied adult are completed. Late in the summer or in the fall the pupal skin breaks and the adult issues. It works its way to the surface of the ground, and betakes itself to the nearest water.

The water-scavenger beetle shows in its post-embryonal development a "complete metamorphosis" as contrasted with the "incomplete metamorphosis" of the locust. Wherever among insects similar changes occur, the young issuing from eggs as larvae only remotely resembling the parent, and these active feeding larvae changing finally into more or less quiescent, strictly non-feeding pupae, which finally change into the active adults, a complete metamorphosis is said to exist. All the beetles, the b.u.t.terflies and moths, the two-winged flies, the ants, bees and wasps, and certain other groups of insects undergo in their post-embryonic development a complete metamorphosis. The crickets, katydids, the sucking bugs, the May-flies, the white ants and numerous other insects have, like the locust, an incomplete metamorphosis, that is, the young when hatched resemble in most respects, except in the absence of wings, their parents.

The adult water-scavenger beetle feeds chiefly on decaying vegetation in the water, but instances of the taking of other insects and of snails have been noted. Although an aquatic insect the beetle, like its larva, has no gills for breathing the air which is mixed with the water, but has to come to the surface occasionally to obtain air. This it does in an interesting way, which should be carefully observed by the pupils. The air is received and held by a covering of fine hairs on the ventral surface of the body, so that a considerable supply may be carried about by the beetle while underneath the surface. The beetles often leave the water by night, flying abroad to other ponds or streams. In winter the beetles hibernate, burying themselves in the banks of the ponds which they inhabit.

For a good account, with ill.u.s.trations, of the water-scavenger beetle's life-history see Miall's "Natural History of Aquatic Insects," pp. 61-87.

THE MONARCH b.u.t.tERFLY (_Anosia plexippus_)

TECHNICAL NOTE.--The Monarch or Milkweed b.u.t.terfly is distributed all over the country. It is large, and red-brown in color, and lays its eggs on milk weeds where the greenish yellow and black-banded larvae (caterpillars) may be found feeding. The covering of scales conceals the outlines of the various external parts, but these scales may be easily removed with dissecting needle and a small brush. In brushing the scales from the head care must be taken not to break off the mouth-parts.

=External structure= (fig. 41).--Note the three body-regions, _head_, _thorax_ and _abdomen_. Is the body segmented? Note the dark color and firm character of the chitinized cuticle.

[Ill.u.s.tration: FIG. 41.--Body of the monarch b.u.t.terfly, _Anosia plexippus_, with scales removed to show the external parts.]

Note on the head the large _compound eyes_. Note the tumid convex _clypeus_ which composes most of the anterior aspect of the head. Are _ocelli_ present? Compare the _antennae_ with those of the locust and water-beetle. Compare also the _mouth-parts_ and note that they differ radically from those of the locust and beetle. They are not fitted for biting, but for sucking up liquid food (the nectar of flowers). Note the absence of a movable flap-like _labrum_ (a minute narrow stiff piece, bearing at each latera end a small group of fine brown hairs, represents the labrum), the entire absence of _mandibles_, and the absence of a movable flap-like labium. The _labium_ is a fixed chitinized triangular piece forming part of the floor of the head.

Note the long slender _proboscis_ coiled up like a watch-spring. (In fresh specimens this proboscis can be uncoiled and will be found flexible. If dried or alcoholic specimens are being studied, the head of the b.u.t.terfly should be removed and softened in warm water before the mouth-parts are examined.) On either side of this proboscis is a peculiar pointed process which rises from the under side of the head.