Part 10

The mouth is a very complicated organ in the crabs, and equally so in the insects. It is generally separated into four distinct parts: the upper lip, labrum, the jaws, or mandibles, a second pair of jaws smaller than the above, and the lower lip or jaws, labium. These are formed into sucking organs in the mosquito, biting organs in the ant, and tremendous graspers in the centipede, all displaying the most remarkable variety.

The eyes of insects are wonderful organs, being both simple and compound. In the grasshopper (Fig. 155) the two are easily seen, the compound eye being the larger. The fly has a remarkable compound eye (Fig. 156), and in the center of the two eyes are three simple ones. The compound eye in the fly is made up of vast numbers of six-sided eyes crowded together, appearing under a glass like a honeycomb; yet each of these facets is a complete eye. In a sectional view of the eye of a beetle (Fig. 157) we can see the nerve that reaches every one. In using the eyes hundreds of images of the same object must reach the brain of the insect, yet the image of but one is seen.

Attached to the head of insects are various sense organs, feelers, or antennæ, which are very ornamental, as in the beetles. The central portion of the skeleton bears the wings. In the beetle the wing covers are formed of hard chitin. When its wings are not in use this insect stores them away in covers provided for the purpose.

The third or last part of the skeleton, the abdomen, often bears a weapon of defense, as a sting or a drill for boring holes in trees, or machines for making silk, as in the spiders. Here also we find a remarkable variety of tails, ranging from that of the dragon fly to the long tail of the scorpion with its dangerous sting or dagger.

The feet of insects would make an interesting chapter alone, ranging from the curious, sucking, padded foot of the fly (Fig. 158) to the claws of others. The anatomy of insects is more or less complicated. The method of breathing is particularly interesting. It is very natural to imagine all animals breathing by the mouth or nostrils, but insects breathe by a singular system of air tubes (Fig 159), or tracheæ, some having lungs as well. The air tubes are wound with threads upon the inside. This preserves their shape and keeps them open. If we examine a grasshopper (Fig. 160), we shall see along the sides openings, which under a powerful microscope resemble eyelike organs (Fig. 161). These are air holes, windows, or spiracles, which lead to the air tubes, and by minute thread-lined tubes reach all over the body. To obtain air, or to breathe, the bee keeps its abdomen continually in motion, forcing air through the body, carrying oxygen to the blood tissues.

The insects, with some exceptions, deposit eggs, and the young pass through many strange changes, or metamorphoses, before the full-grown form is attained. The eggs of beetles hatch into larvæ (Fig. 162), which may live weeks or months or even years in the ground. The eggs of other insects, as moths, may become caterpillars, which finally spin a cocoon, as in the case of the silkworm, and from this cell-like room appears the perfect moth. These changes, so infinite in their variety, are among the most interesting features of insect life, and are never failing sources of wonder and amazement on the part of those who devote time to the study.

XIX. LOWER FORMS OF INSECTS

In the previous chapter we have by the aid of illustrations glanced at the structure of insects, and noted some of the features which distinguished them from the crabs. Now we may take up some of the more important and interesting groups and observe how Nature has adapted them to their peculiar surroundings, and for the various offices they fill in the world.

In examining the various families of insects it is interesting to note that many produce certain results or accomplish certain ends in totally different ways. The highly organized spider by drawing silk from the spinning machines at the tip of its abdomen builds a web so strong that it sometimes captures birds. I recently found a living humming bird hard and fast in a web at the corner of my house, and released it just in time to save it from the spider. In the very lowest groups of insects we find the Peripatus (Fig. 163), which spins a weblike structure from glands in its mouth, ejecting the secretion at the insect it wishes to catch. This appears to crystallize in the air and hold the victim despite its struggles. The Peripatus, found in Africa and Central America, resembles a large caterpillar, having a long, soft, cylindrical body with many pairs of feet, sometimes sixty-six; the latter are soft and armed with claws. It is remarkable for the possession of many legs, but is outdone in this respect by the millipeds, as some (Fig. 164) have as many as two hundred. These insects, when placed upon their backs, present an extraordinary appearance, clawing the air; yet they are among the slowest of walkers. They live in the ground, are harmless, feed on vegetable matter, and deposit their eggs in the earth, which hatch out little creatures at first resembling crickets.

The centipeds, on the other hand (Fig. 165), are animal feeders, and those found in the tropics are formidable creatures from six to ten inches in length, supplied with many claws and terrible fangs. They live a life of rapine and destruction, and the appearance of a large specimen almost a foot in length, dashing along with great rapidity by the aid of its fifteen or twenty pairs of feet, is sufficient to demoralize the stroller through the dark glades of the tropical forests. They have two pairs of foot jaws (Fig. 166) which grasp an enemy with wonderful tenacity. The second pair is perforated, and from it pours a poison dangerous to man in some tropical species and fatal to small animals. Several of these hideous creatures are luminous at times. Many centipeds have long antennæ. The eyes are very small, and in groups. The ordinary centiped of the North is harmless, despite the tales related of its ferocity.

Among the very small, though destructive insects, are the mites, found in cheese and sugar; they are parasitic in cattle and various other animals. In California certain forms (Fig. 167) cling to the bushes.

In remarkable contrast to the round-bodied mites are the scorpions (Fig. 168), in which the tail is sometimes two inches in length and armed with a sharp, daggerlike sting, provided with a poison apparatus. The scorpions of the largest size are often found in the tropics in the same locality with centipeds, under board piles and in dark places, coming out at night to prey upon small insects, which they seize with their crablike claws and tear apart. If the insect struggles violently, the scorpion raises its tail over its back and pierces it with its dagger, paralyzing it. In striking at other enemies the scorpion whirls about, keeping its tail toward them, repeatedly striking down and using its jointed tail with marvelous ingenuity. A few years ago these scorpions were common on the Florida Reef and were frequently killed in my own house at night. The pain resulting from the sting was about as disagreeable as that occasioned by a wasp. These scorpions were about three inches in length, but in Ceylon very much larger ones have been seen, and known to catch birds. The young scorpions are born alive and cling to the mother. The little book scorpion, the large whip scorpion, and the daddy longlegs, or harvestman, a harmless and sociable insect, are related to the true scorpions.

XX. THE SPIDERS

A little insect half as large as a grain of corn, finds itself on a limb high above ground and is desirous of reaching another, five feet away. It is not a jumper, at least it could not hope to cover this distance; neither has it wings. But it has a marvelous silk-manufacturing apparatus, known as spinnerets (Fig. 169), and elevating its abdomen it reels off a thread which the wind carries across the chasm where it lodges. Across the single cord the spider runs, the act being suggestive of the intelligence of these insects.

The common garden spider (Fig. 170), which may illustrate the group, is seen to differ very materially from the scorpion. The abdomen is not ringed or made up of segments, but is large and plump, and connected with the thorax by a delicate cord or pedicel.

The spiders, as we have seen, have a spinning arrangement by which they form beautiful webs or nets to capture prey. By this silken cord they can lower themselves from great heights. The single thread which supports them is made up of a number of minute threads (Fig. 171). The webs are formed in endless variety and with all the skill of a bridge maker, being guyed, supported, and braced in a manner which, if the work of man, would be said to be the result of endless study. They are perfect in their arrangement, and each web is a study in geometry, yet the spider builds it with the greatest rapidity, never hesitating in the making or repairing.

By my door is a huge spider similar to the one shown in Figure 172. It has a beautiful web which covers a space two feet square, but the spider rarely occupies it. Near by it has a covering formed of a leaf of a fern which it has pulled down each side and fastened, forming a little room just the size of its body. Wondering how the spider would discover a victim caught in the web, I examined it carefully, and then placed a grasshopper in the web. Instantly the spider noted the disturbance, having what to all intents and purposes was a private telephone line. This was a single guy line leading from the center of the web to its retreat, where one of the spider's claws rested upon it, holding it, so that the slightest swaying of the web lifted its foot. When an insect became entangled, the spider darted at it, and by skillful manipulation of its hind pair of legs reeled off its silken cord and attached it to the victim at every point, in a short time literally binding it in a roll. If it was likely to escape, the spider would bite it, using its poison fang (Fig. 173), which paralyzed it. The biting mandibles (Fig. 174) are terrible weapons, from which there is no escape. The inner jaws (Fig. 175) are equally sharp and effective. The eyes of the spider are very brilliant, and in a bright light can be seen to gleam and glisten like points of steel or fire. They are minute dots, seen just above the mandibles.

The male and female spiders often present a very different appearance, the male being smaller. The spiders deposit eggs which are inclosed in silken balls or nests of various kinds, in which they remain until the young are hatched. Some are concealed in the web; others are placed underground; and some are perched upon a stalk resembling a plant.

There appears to be no limit to the uses to which the marvelous silk of spiders is put. Some spiders form balloons with which they sail away through the air. I have seen scores of these aëronauts in the air at one time. Another form constructs a raft of leaves bound together with silk. Some build nets for small game, as gnats. The silken cord made by others is so tough that it can be used as thread.

By partly destroying a web and suspending a black cloth behind the locality, the operations of the spider in building and repairing can be plainly seen. It is well to place the spinnerets beneath a microscope, under which they appear to be made up of many points. Touch one of these and a glutinous secretion adheres, which when stretched is seen to be silk, and each point provides a separate thread which joins with the others, producing one cable. The spinnerets are to some extent movable. They can be turned to the right or left, and wherever they touch, the silk remains glued fast. This explains why the spider moves and works so quickly and accomplishes so much. The amount of silk secreted is astonishing, and some idea of it can be obtained by walking over the country in spring, early in the morning. On the slopes of the Sierra Madre, in the San Gabriel Valley, I have seen the surface of the ground for a great distance covered with webs which caught the rays of the sun as it rose, presenting a most beautiful appearance. This fabric covered hundreds of acres in a fairy maze of web, so many traps for unwary small fry of the insect world. With a small stick I have wound a large amount of silk from the spinnerets of a spider, there apparently being no diminution of the supply. Professor Burt Wilder wound from the large spider known as Nephila plumipes several miles of silk. Some spiders have long, slender legs and are rapid runners. Others, as Salticus, are very deliberate, but powerful leapers, jumping upon their prey like a cat. Perhaps the most remarkable leaping spider is one from Australia, called the flying Attus, having singular flaps or winglike extensions upon its sides. One of the spiders not only runs over the surface of the water readily, but spends a part of its time under the surface, carrying down a bubble of air for its supply of oxygen, the bubble acting as a diving bell.

The spiders are very solicitous of their young, placing every safeguard about them, and resenting any attack by a fierce rush. Several large spiders (Fig. 176) carry their young upon their backs. The little spiders are rubbed or scraped off when they become too great a burden. The spiders are natural hunters and trappers, and a volume could be written on their methods and adventures in running down prey. Once as I was crawling through the almost stifling brush of one of the Florida Keys I came to a little opening about five feet wide, across which was a large, conspicuous, and powerful web. In the center of this web clung a huge and most remarkable spider, colored a vivid yellow and black. I watched it for a few moments while resting, and then touched the web, whereupon the spider began to swing, by raising and depressing its body, increasing its speed rapidly, until I could with difficulty see it. A moment later it disappeared almost entirely before my eyes. For half a minute the spider kept up this motion, then it slowly came to a standstill, having demonstrated that it could easily disappear from any bird enemy without running away. I have seen the daddy longlegs perform the same feat in California.

The spiders which build webs, from plain geometrical traps to conelike affairs, are interesting; but the trapdoor spiders and those which dig burrows are among the wonderful artisans and engineers of the insect world. One of the most perfect doors, in hinge, fit, beauty of interior, finish, and quality of its outward defense is built by a spider (Fig. 177). In the illustration, the den and its trapdoor is shown, and in Figure 178 a sectional view of the same is seen; but the door is never found open, the spring or hinge being so devised as to remain closed. I have found many of these dens in southern California sometimes a foot or more in length. The spider is not the large one shown in the cut, which is a tarantula, but is very much smaller, though a large spider. The genera, Cteniza and Nemesia, are best known for their cunning and skill as builders. The California spider begins its den when very small, and I have found many the size of goose quills, with door complete, in the vicinity of a large den.

In forming the burrow the spider carries out the clay bit by bit, and when it reaches a point below the surface it begins to line the sides with a silken tapestry. The door is an upper extension of this lining. It is round, about the size of a silver quarter, or a little larger, and is formed of silk so woven and interwoven that it becomes a pad of seeming satin, which by continued manipulation is made to fit with marvelous perfection. The spring or hinge is so adjusted that the door always closes, and with a snap. The exterior of the door is covered with clay, and is made to simulate the surroundings so exactly that only the sharpest eye, and one skilled in the work, can distinguish it. In some of the European spiders of this kind the door is carefully covered with moss and plants. The work of building is done at night. The spiders feed at night, and in returning to the burrow they can lift the lid instantly, dart in, and turn about to seize the cushion or pad of the door with their fangs, and hold it so tightly by bracing back that some little strength is needed to force it. I have often lifted the door with the blade of my knife and seen the spider rush up and seize it. In all the doors little round holes can be seen where the mandibles or fangs hold. The spider can be caught by pouring water into the burrow and forcing it out. In the island of Timos there is a trapdoor spider which does not hunt, but combines the methods of other spiders. It comes out at night, fastens back its door by a thread, then builds a web near by and waits for its victims to become entangled.

The largest spiders are called tarantulas, though the term is applied to some forms not so large. They are hideous creatures, and are very common in southern California. They are five or six inches across the legs, and the body in some forms is as large as a small mouse, and is covered with reddish hair. They form deep burrows, but not trapdoors, the entrance being open or covered by a web. They are supposed to be very poisonous. Mygale Hentzii is the name of the common American species found in the southwest (Fig. 177). They hunt at night for grasshoppers, crickets, and other small game, and can often be seen lumbering along over the roads at sunrise, returning from a hunt. Sometimes these huge spiders migrate in a body, such a movement having been observed in southern California.

A South American species has been known to attack and capture small birds, though this may be considered rather the exception than the rule, their food consisting of large insects and small lizards. Of all the spiders, the Tarantula turricula (Fig. 179) is the most remarkable, as it not only makes a deep burrow, but erects above it a chimneylike structure with all the skill of a human workman. Indeed, the structure, in neatness and perfection of design, is far superior to many of the chimneys seen among the poorer classes of some countries. The spider lays the miniature timbers across with the precision and exactness of a skilled carpenter and after the manner of human log-cabin builders. The female carries her young upon her back, as shown in the illustration.

XXI. SOME SIX-LEGGED INSECTS

One of the great divisions into which the insects are divided relates to their possession of six legs (Fig. 180). This includes a marvelous array of creatures. Among them we find the singular little glacier flea (Fig. 181) and the springtail, a prodigious jumper (Fig. 182). The latter is found in damp places, and when touched will release a forked spring which is held in place by a hook, and this sends the insect flying into the air like an acrobat. These humble little creatures present a strange contrast to the lace-winged insects which are among the most beautiful of the tribe.

The May flies (Fig. 183) are well known for the wonderful exhibitions they sometimes make, the air being filled with them, a joyous, beautiful throng, destined to live but a few hours. In South America they occur in such vast numbers that they are collected and used as guano. The young pass through a strange change, having little resemblance to the parent, and live in the water, where they breathe by means of several plumelike gills.