Chapter XVI: Introduction: Habits–classification–structure–chilognatha–chilopoda (10)

The hind body is elongate, and shows distinctly eight dorsal segments, behind which are the pieces forming—in the female, the fossorial organs which replace an ovipositor—in the male, the modified parts connected with the terminal segment. The alar organs (Fig. 167) exhibit, according to Brunner, the same areas as we have described in Blattidae. According, however, to Redtenbacher[210] the tegmina of the Acridiidae and other saltatorial Orthoptera differ from those of the cursorial group (with the exception of the Phasmidae) in that they possess a praecostal field, due to the fact that the vein which in the Cursoria is costal, _i.e._ forms the front margin, in the Saltatoria lies, on the contrary, in the field of the wing. If this view be correct the mediastinal area of Brunner is not homologous in the two divisions. The tegmina are long and comparatively narrow; they are of firm parchment-like texture, with several longitudinal veins, which divide beyond the middle, so as to become more numerous as they reach the extremity of the wing; there is much reticulation, dividing the surface into numerous small cells. The hind wings are much more ample, and of more delicate texture; the longitudinal veins fork but little, the numerous cross veinlets are fine. In repose the hind wings fold together in a fan-like manner, and are entirely concealed by the upper wings. The front and middle legs are similar and small, the coxae are quite small, and do not completely fill the articular cavities, which are partly covered by membrane; all the tarsi are three-jointed. The basal joint, when looked at beneath, is seen to bear three successively placed pads, so that from beneath the tarsi look as if they were five-jointed {282}(Fig. 185, C). The hind legs are occasionally very long; their femora, thicker towards the base, are generally peculiarly sculptured, bearing longitudinal ridges or grooves, which are more or less spinose, and are also very frequently marked with short parallel lines meeting a central longitudinal line at similar angles, so as to give rise to a well-marked pattern; where the legs are broader the pattern is more complex (Fig. 168). The long tibiae bear two rows of spines on their upper or posterior edge; this part of the hind leg can be completely bent in under the femur. The stigmata consist of one prothoracic, one metathoracic, and eight abdominal pairs.

In reference to the ocelli, which are shown in Fig. 166, we may remark that the Acridiidae is one of the large groups of Insects in which the coexistence of compound and single eyes is most constant, though in some of the wingless forms the ocelli are very imperfect. We know at present of nothing in the habits of Acridiidae to render two kinds of eyes specially necessary. We shall subsequently see that a similar condition in regard to the function of hearing is believed to exist in this family.

Acridiidae are remarkable amongst the Orthoptera for the possession of air sacs or vesicular dilatations in the interior of the Insect in connexion with the tracheae (Fig. 176). Such vesicles are found in many of the higher winged Insects, but not in larval forms, or in those that are destitute of powers of flight.[211] They, no doubt, assist the Insect in its movements in the air. The body of a large grasshopper or locust is naturally of considerable weight, and it is more than probable that true flight can only be accomplished when these vesicles are dilated and filled with air. The exact mode in which the sacs are dilated is not known; possibly it may be accomplished by the elasticity of the structure of the vesicles coming into action when the other contents of the {283}body are not completely developed, or are temporarily diminished. Although air vessels are absent in the neighbouring groups of Orthoptera, Dufour says they are present even in apterous forms of Acridiidae, but he gives no particulars.[212] Packard has given an account[213] of the arrangement of these remarkable sacs in the Rocky Mountain Locust. He finds that there are two sets: a thoracic group, consisting of a pair of very large size, with which are connected some smaller sacs placed in the head; and an abdominal set, which forms a very remarkable series. The figures we give (Fig. 176, A, B) show that these sacs are of such large size that if fully distended they must interfere with the development of the ovaries, and that they must be themselves greatly diminished, if not obliterated, by the distension of the alimentary canal. We may look on them as only coming into full play when the normal distension of the canal is prevented, and there is only small development of the reproductive organs. Under such circumstances the locust becomes a sort of balloon, and migrates. In addition to the air sacs there are many dilatable tracheae, placed chiefly in parts of the body where there is not space for the large air sacs. These are, for the sake of clearness, omitted from our figure.

The ganglia constituting the brain are simpler in Acridiidae than they are in the higher Insects, such as bees and wasps, and have been specially studied by Packard[214] and Viallanes.[215] The other ganglia of the nervous cord are eight in number, three thoracic and five abdominal.

The salivary glands are small. The alimentary canal is capacious but not coiled. It has no gizzard, but the crop has a peculiar structure, apparently as a substitute. There are diverticula connected with the true stomach. The Malpighian tubes are elongate {284}and extremely numerous. The pair of testes is united in a single envelope. The form and arrangement of the ovaries is remarkable (Fig. 169); the egg-tubes are united by the convergence of their terminal threads into a single mass; outside of each ovary there extends a large calyx, into which the tubes open; each calyx is prolonged at its extremity, and forms a long, convoluted tube.

Acridiidae possess structures for the production of sound, together with others that are, no doubt, for hearing. The chirping of grasshoppers is accomplished by rubbing together the outer face of the upper wing and the inner face of the hind femur. This latter part bears a series of small bead-like prominences placed on the upper of the two lower ridges that run along the side that is nearest to the body (Fig. 170); the tegmen or wing-case has projecting veins, one of which is slightly more prominent, and has a sharp edge; by scraping this edge over the beads of the femur the wing is thrown into a state of vibration and a musical sound is produced. The apparatus for producing sound was for long supposed to be confined to the male sex of grasshoppers; it was indeed known that females made the movements appropriate for producing music, but as they appeared to be destitute of instruments, and as no sound was known to follow from their efforts, it was concluded that these were merely imitative. Graber has, however, discovered[216] that rudimentary musical organs do exist in the females of various species of _Stenobothrus_ (Fig. 171, B). It is true that in comparison with those of the male (Fig. 171, A) they are minute, but it would appear that they are really phonetic, though we can hear no sounds resulting from their use.

Graber considers that the musical pegs of Acridiidae are {285}modified hairs, and he states that in certain females the stages intermediate between hair and peg can be found. There is apparently much variety in the structure of these instruments in different species, and even in individuals of the same species. In _Stenobothrus lineatus_, instead of pegs, the instrument consists of raised folds.

In some of the aberrant forms of Acridiidae—certain Eremobiides and Pneumorides—the males are provided with sound-producing instruments different to those we have described, both as regards situation and structure.

If the dorsal aspect of the first segment of the hind body of an Acridian Insect be carefully examined there may be seen in the majority of species an organ which has somewhat the appearance of an ear (Fig. 172), and which there is great reason for believing to be really an organ of that nature. It is situate a little over the articulation of the hind leg, very close to the spot where the sound is, as above described, produced. There are three forms of these Acridian ears as described by Brunner:[217] (1) a membrane surrounded by a rim; (2) the membrane somewhat depressed, a portion of the segment projecting a little over it; (3) the depression very strongly marked, and the sides projecting over it so much that all that is seen externally is a sort of broad slit with a cavity beneath it. This last is the condition in which the ear exists in the genera _Mecostethus_ (Fig. 173) and _Stenobothrus_, which are among our few native grasshoppers. On minute examination this ear proves to consist of a tympanum supplied internally with nerve and ganglion in addition to {286}muscles, and tracheal apparatus of a complex nature; it is no doubt delicately sensitive to some forms of vibration. Unlike the stridulating organ, these ears exist in both sexes; they are found in a great majority of the species of Acridiidae. The forms in which the ears are absent are usually at the same time wingless and destitute of organs of stridulation; but, on the other hand, there are species—some of them wingless—that are, so far as is known, incapable of stridulation and yet possess these ears.

It is, indeed, a matter of great difficulty to decide as to the exact function of these ear-like acoustic organs, which, we may remind the reader, are peculiar to the saltatorial Orthoptera, and we must refer for a full discussion of the subject to Graber's masterly works,[218] contenting ourselves with a brief outline, which we may commence by saying that the Orthoptera with ears are believed to be sensitive to sounds by means other than these organs. This suggests that the latter exist for some purpose of perception of special sound. But if so what can this be? Only the males possess, so far as we know, effective sound-producing organs, but both sexes have the special ears; moreover, these structures are present in numerous species where we do not know of the existence of phonetic organs in either sex. Thus it appears at present impossible to accept these organs as being certainly special structures for the perception of the music of the species. It is generally thought that the females are charmed by the music of the males, and that these are stimulated to rivalry by the production of the sounds; and Dufour[219] has suggested that this process reacts on the physiological processes of the individual. There has not been a sufficient amount of observation to justify us in accepting these views, and they do not in any way dispose of the difficulty arising from the existence of the acoustic organs in species that do not, so far as we know, produce special sounds. It is possible that the solution of the difficulty may be found in the fact that these apparently dumb species do really produce some sound, though we are quite ignorant as to their doing so. It is well known that sounds inaudible to some human ears are perfectly distinct to others. Tyndall, in his work on Sound, has illustrated this by a fact that is of special interest from our present point of view. "Crossing {287}the Wengern Alp with a friend," he says, "the grass on each side of the path swarmed with Insects which to me rent the air with their shrill chirruping. My friend heard nothing of this, the Insect world lying beyond his limit of audition." If human ears are so different in their capacities for perceiving vibrations, it of course becomes more probable that auditory organs so differently constituted as are those of Insects from our own may hear sounds when the best human ear can detect nothing audible. On the whole, therefore, it would appear most probable that the Orthoptera provided with acoustic organs, and which we consider dumb, are not really so, but produce sounds we cannot hear, and do so in some manner unknown to us. If this be the case it is probable that these ears are special organs for hearing particular sounds.

Scudder, who has given considerable attention to the subject of Orthopteran music, says that in N. America "the uniformity with which each species of _Stenobothrus_ plays its own song is quite remarkable. One kind, _Stenobothrus curtipennis_, produces about six notes per second, and continues them from one and a half to two and a half seconds; another, _S. melanopleurus_, makes from nine to twelve notes in about three seconds. In both cases the notes follow each other uniformly, and are slower in the shade than in the sun."

Some of the species of Acridiidae, it should be noticed, produce a noise during their flights through the air, due to the friction of the wings; whether this has a definite importance, or whether it may be entirely incidental, has scarcely yet been considered.

Information of a satisfactory kind as to the post-embryonic development of the Acridiidae is but scanty. We have represented in Fig. 84, A, the condition in which a migratory locust, _Schistocerca peregrina_, leaves the egg, and we will here complete the account of its growth; following Brongniart,[220] whose statement is confirmed by Lestage and other naturalists. Immediately on leaving the egg the young locust casts its skin, and is then of a clear green colour, but it rapidly becomes brown, and in twelve hours is black. At this early age the gregarious instinct, possessed by this and some other species of Acridiidae, becomes evident. In six days the individual undergoes a second moult, after which it is black, spotted and banded with white, and with a rose-coloured streak on each side of the hind body. The {288}third ecdysis occurs in six or eight days after the second; the rose colour becomes more distinct, and the head is of a brown tint instead of black. After eight days the fourth ecdysis occurs; the creature is then about 35 millimètres long; its colour has much changed, the position of the markings is the same, but the rose colour is replaced by citron yellow, the line of the spiracles is marked with white, and at this time the creature has the "first rudiments of wings," and is very voracious. In ten days another ecdysis takes place, the yellow colour is more vivid, the prothorax is definitely speckled with white, and the hind body is increasing much in size. In fifteen or twenty days the sixth moult occurs, and the Insect appears in its perfect form; the large tegmina now present are marked with black in the manner so well known, and the surface generally is variegated with bluish and rosy marks. Although this is the colour in Algeria, yet apparently it is not so farther south; the Insects that arrive thence in the French colony are on some occasions of a different colour, viz. reddish or yellowish, those of this latter tint being, it is believed, older specimens of the reddish kind. M. Brongniart points out that some Phasmidae—of the _Phyllium_ group—undergo an analogous series of colour-changes in the course of the individual development, though other species do not.

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Riley and Packard have given an account[221] of some parts of the post-embryonic development of the Rocky Mountain Locust, which enables us to form a satisfactory conception of the stages of development of the wings. Fig. 175, A, represents the first instar, the young locust, just emerged from the egg and colourless. Fig. 174 shows some of the subsequent stages of development of the wings, the upper line of figures giving a profile view of the thoracic segments, and the lower line showing their dorsal aspects; 1 shows the condition of the parts in the second instar, the chief difference from the first instar being the development of colour; in the third instar there is an evident slight development of the future alar organs, exhibited chiefly in the outgrowth and lobing of the free posterior angles of the meso- and metanota, as shown in Fig. 174, 2. After the third moult there is a great difference; the instar then disclosed—the fourth—has undergone a considerable change in the position of the meso- and metathoraces, which are thrust forward under the pronotum; this has become more enlarged and hood-like (Fig. 174, 3); at the same time the wing-rudiments have become free and detached, the metathoracic pair being the larger, and overlapping the other pair. The fifth instar (Fig. 174, 4) differs but little from the fourth, except in the larger size of the pronotum and wing-rudiments. The sixth—shown in Fig. 175, {290}B—is the perfect Insect, with the alar organs free and large, the prothorax much changed in form, the colour different. From the above it will be seen that the chief changes occurred at the third and fifth ecdyses, after each of which a considerable difference in the form of the Insect was revealed. In the first three instars the sexes can scarcely be distinguished, in the fourth they are quite distinct, and in the fifth coupling is possible, though usually it does not occur till the final stage is attained.

The discovery that Orthoptera change their colours in the course of their development, and even after they have become adult, is important, not only from a physiological point of view, but because it throws some light on the questions as to the number of species and the geographical distribution of the migratory locusts, as to which there has existed a great confusion.

The Acridiidae are considered to be exclusively vegetable feeders, each individual consuming a very large quantity of food. The mode in which the female deposits her eggs has been described by Riley,[222] and is now widely known, his figures having been frequently reproduced. The female has no elongate ovipositor, but possesses instead some hard gonapophyses suitable for digging purposes; with these she excavates a hole in the ground, and then deposits the eggs, together with a quantity of fluid, in the hole. She prefers hard and compact soil to that which is loose, and when the operation is completed but little trace is left of it. The fluid deposited with the eggs hardens and forms a protection to them, corresponding to the more definite capsules of the cursorial Orthoptera.

The details of the process of oviposition and of the escape of the young from their imprisonment are of much interest. According to Künckel d'Herculais[223] the young _Stauronotus maroccanus_ escapes from the capsule by putting into action an ampulla formed by the membrane between the head and the thorax; this ampulla is supposed to be dilated by fluid from the body cavity, and is maintained in the swollen condition by the Insect accumulating air in the crop beneath it. In order to dislodge the lid of the capsule, six or seven of the young ones inside combine their efforts to push it off by means of their ampullae. The ampulla {291}subsequently serves as a sort of reservoir, by the aid of which the Insect can diminish other parts of the body, and after emergence from the capsule, penetrate cracks in the earth so as to reach the surface. Immediately after doing this the young _Stauronotus_ moults, the skin it casts being called by Künckel an amnios. The cervical ampulla reappears at subsequent moults, and enables the Insect to burst its skin and emerge from it.

The process is apparently different in _Caloptenus spretus_, which, according to Riley, ruptures the egg-shell and works its way out by the action of the spines at the apex of the tibiae. This latter Insect when it emerges moults a pellicle, which Riley considers to be part of the embryonic membranes.

Riley states that a female of _Caloptenus spretus_ makes several egg-masses. Its period of ovipositing extends over about 62 days, the number of egg-masses being four and the total number of eggs deposited about 100. The French naturalists have recently observed a similar fact in Algeria, and have ascertained that one of the migratory locusts—_Schistocerca peregrina_—may make a deposit of eggs at more than one of the places it may alight on during its migration.

It has been ascertained that the eggs of Acridiidae are very nutritious and afford sustenance to a number of Insects, some of which indeed appear to find in them their sole means of subsistence. Beetles of the family Cantharidae frequent the localities where the eggs are laid and deposit their eggs in the egg-masses of the Orthoptera, which may thus be entirely devoured. Two-winged flies of the family Bombyliidae also avail themselves of these eggs for food, and a mite is said to be very destructive to them in North America. Besides being thus destroyed in enormous quantities by Insects, they are eaten by various birds and by some mammals.

Most of the Insects called locusts in popular language are members of the family Acridiidae, of which there are in different parts of the world very many species, probably 2000 being already known. To only a few of these can the term Locust be correctly applied. A locust is a species of grasshopper that occasionally increases greatly in number, and that moves about in swarms to seek fresh food. There are many Orthoptera that occasionally greatly increase in numbers, and that then extend their usual area more or less; and some Acridiidae multiply {292}locally to a great extent—very often for one or two seasons only,—and are then called locusts. The true migratory locusts are species that have gregarious habits strongly developed, and that move over considerable distances in swarms. Of these there are but few species, although we hear of their swarms in many parts of the world.

The migratory locusts do much more damage than the endemic species. In countries that are liable to their visitations they have a great influence on the prosperity of the inhabitants, for they appear suddenly on a spot in huge swarms, which, in the space of a few hours, clear off all the vegetable food that can be eaten, leaving no green thing for beast or man. It is difficult for those who have not witnessed a serious invasion to realise the magnitude of the event. Large swarms consist of an almost incalculable number of individuals. A writer in _Nature_[224] states that a flight of locusts that passed over the Red Sea in November 1889 was 2000 square miles in extent, and he estimated its weight at 42,850 millions of tons, each locust weighing 1/16 of an ounce. A second similar, perhaps even larger, flight was seen passing in the same direction the next day. That such an estimate may be no exaggeration is rendered probable by other testimony. From official accounts of locusts in Cyprus we find that in 1881,[225] up to the end of October, 1,600,000,000 egg-cases had been that season collected and destroyed, each case containing a considerable number of eggs. By the end of the season the weight of the eggs collected and made away with amounted to over 1300 tons, and, notwithstanding this, no less than 5,076,000,000 egg-cases were, it is believed, deposited in the island in 1883.

When we realise the enormous number of individuals of which a large swarm of locusts may consist we can see that famine is only a too probable sequence, and that pestilence may follow—as it often has done—from the decomposition of the bodies of the dead Insects. This latter result is said to have occurred on some occasions from locusts flying in a mass into the sea, and their dead bodies being afterwards washed ashore.

Locust swarms do not visit the districts that are subject to their invasions every year, but, as a rule, only after intervals of a considerable number of years. It has been satisfactorily {293}ascertained that in both Algeria and North America large swarms occur usually only at considerable intervals. In North America Riley thought[226] the average period was about eleven years. In Algeria the first invasion that occurred after the occupation of the country by the French was in 1845, the second in 1864, the third in 1866, since which 1874 and 1891 have been years of invasion. These breaks seem at first strange, for it would be supposed that as locusts have great powers of increase, when once they were established in any spot in large numbers, there would be a constant production of superfluous individuals which would have to migrate as regularly as is the case with swarms of bees. The irregularity seems to depend on three facts: viz. that the increase of locusts is kept in check by parasitic Insects; that the eggs may remain more than one year in the ground and yet hatch out when a favourable season occurs; and that the migratory instinct is only effective when great numbers of superfluous individuals are produced.

It is not known that the parasites have any power of remaining in abeyance as the locust eggs may do; and the bird destroyers of the locusts may greatly diminish in numbers during a year when the Insects are not numerous; so that a disproportion of numbers between the locusts and their destroyers may arise, and for a time the locusts may increase rapidly, while the parasites are much inferior to them in numbers. If there should come a year when very few of the locusts hatch, then the next year there will be very few parasites, and if there should then be a large hatching of locusts from eggs that have remained in abeyance, the parasites will not be present in sufficient quantity to keep the destructive Insects in check; consequently the next year the increase in number of the locusts may be so great as to give rise to a swarm.

It is well established that locusts of the migratory species exist in countries without giving rise to swarms, or causing any serious injuries; thus _Pachytylus cinerascens_—perhaps the most important of the migratory locusts—is always present in various localities in Belgium, and does not give rise to swarms. When migration of locusts does occur it is attended by remarkable manifestations of instinct. Although several generations may elapse without a migration, it is believed that the locusts when {294}they migrate do so in the direction taken by predecessors. Their movements are to a large extent dependent on the wind, and it is said that they make trial flights to ascertain its direction. When on the wing probably very little muscular effort is necessary. Their bodies contain elastic air sacs in communication with the tracheae, and at the time of flight it may be presumed that the body is comparatively empty, food being wanting, and the internal organs of reproduction, which occupy a large space when in activity, yet undeveloped, hence the sacs have full room for expansion, as explained on p. 283. Thus the Insects exert but little effort in their aerial movements, and are, it is believed, chiefly borne by the wind. Should this become unfavourable it is said that they alight and wait for a change.

The most obscure point in the natural history of the migratory locusts appears to be their disappearance from a spot they have invaded. A swarm will alight on a locality, deposit there a number of eggs, and then move on. But after a lapse of a season or two there will be few or none of the species present in the spot invaded. This appears to be partly due to the young locusts dying for want of food after hatching; but in other cases they again migrate after growth to the land of their ancestors. The latter fact is most remarkable, but it has been ascertained by the U.S. Entomological Commission that these return swarms do occur.

In South Africa it would appear that the movements of the migratory locusts are frequently made before the Insects have acquired their wings. Mrs. Barber, in an account of "Locusts and Locust-Birds in South Africa,"[227] has illustrated many points in the {295}Natural History of these Insects. The South African species manifests the gregarious and migratory disposition when the individuals are quite young, so that they travel in flocks on foot, and are called by the Dutch "Voetgangers." After hatching, the various families of young amalgamate, so that enormous numbers come together. Having denuded the neighbourhood of all its food-supplies, they move off in search of fresh crops and pastures new. They take advantage of roads, and sometimes a good many miles will be traversed in a day; they proceed by means of short leaps, rapidly repeated. When the "Voetgangers" are thus returning northwards towards the lands in the interior from which their progenitors departed, no obstacles can stay their course. Forests or rivers may intervene, diverting them for a while from their line of march, but they succeed ultimately in continuing their journey to the interior.

The manner in which these wingless locusts occasionally cross broad rivers is interesting, as it has some bearing on the difficult question of the possibility of winged locusts crossing seas of considerable width. Mrs. Barber refers to an instance that took place on the Vaal River in the spring of the year 1871, shortly after the discovery of the Diamond-fields. The country was at that time swarming with young locusts; every blade of grass was cleared off by them. One day a vast swarm of the "Voetgangers" made their appearance on the banks of the Vaal River; they appeared to be in search of a spot for crossing, which they could not find, the river being somewhat swollen. For several days the locusts travelled up the stream; in the course of doing this they paused for some time at an abrupt bend in the river where a number of rocks were cropping out, as if in doubt whether to attempt a passage at this place. They, however, passed on, as if with the hope of finding a better ford; in this apparently they were disappointed, for three days afterwards they returned to the same bend of the river, and there plunged in vast multitudes into the stream, where, assisted by a favourable current and the sedges and water-plants which grew upon the projecting rocks, they managed to effect a crossing, though great numbers were drowned and carried away by the flooded river. Mrs. Barber adds that "Voetgangers" have been known to attempt the passage of the Orange River when it was several hundred yards in breadth, pouring their vast swarms into the flooded stream regardless of the consequences, until they {296}became heaped upon each other in large bodies. As the living mass in the water accumulated, some portions of it were swept away by the strong current from the bank to which they were clinging, and as the living locusts tightly grasped each other and held together, they became floating islands, the individuals continually hopping and creeping over each other as they drifted away. Whether any of the locust-islands succeeded in reaching the opposite bank is unknown; probably some of them were drifted on land again. They are by no means rapid swimmers; they do not perish easily in the water when in masses, their habit of continually changing places and hopping and creeping round and round upon each other being very advantageous as a means of preservation. It is a common practice for the young locusts to form a bridge over a moderately broad stream by plunging indiscriminately into it and holding on to each other, grappling like drowning men at sticks or straws, or, in fact, anything that comes within their reach, and that will assist in floating them; meanwhile those from behind are eagerly pushing forward over the bodies of those that are already in the stream and hurrying on to the front, until at length by this process they reach the opposite bank of the river; thus a floating mass of living locusts is stretched across the stream, forming a bridge over which the whole swarm passes. In this manner few, comparatively speaking, are drowned, because the same individuals do not remain in the water during the whole of the time occupied by the swarm in crossing, the Insects continually changing places with each other; those that are beneath are endeavouring to reach the surface by climbing over others, whilst those above them are, in their turn, being forced below. Locusts are exceedingly tenacious of life, remaining under water for a considerable time without injury. An apparently drowned locust will revive beneath the warm rays of the sun, if by chance it reaches the bank or is cast on shore. Mrs. Barber relates an interesting case where the instinct of the "Voetgangers" was at fault, they plunging into a river from a steep sandy bank, only to find another similar sandy precipice on the other side. On this they could gain no footing, and all perished in the stream, where they putrefied, and caused the death of the fish, which floated likewise on the surface; so powerful were the effluvia produced that no one was able to approach the river.

{297}Locusts are able to travel considerable distances, though how far is quite uncertain. Accounts vary as to their moving by night. It has, however, been recently proved that they do travel at night, but it is not ascertained how long they can remain in the air without descending. The ocean is undoubtedly a source of destruction to many swarms; nevertheless, they traverse seas of considerable width. They have been known to reach the Balearic Islands, and Scudder gives[228] a well-authenticated case of the occurrence of a swarm at sea. On the 2nd of November 1865 a ship on the voyage from Bordeaux to Boston, when 1200 miles from the nearest land, was invaded by a swarm of locusts, the air and the sails of the ship being filled with them for two days. The species proved to be _Acridium_ (_Schistocerca_) _peregrinum_. This is an extraordinary case, for locusts do not fly with rapidity, being, indeed, as we have remarked, chiefly carried by the wind. Possibly some species may occasionally rest on the water at night, proceeding somewhat after the fashion of the "Voetgangers" when passing over rivers as described by Mrs. Barber. In Sir Hans Sloane's history of Jamaica an account of an occurrence of this kind is given on the authority of Colonel Needham, who states that in 1649 locusts devastated the island of Tenerife, that they were seen to come from Africa when the wind was blowing thence, that they flew as far as they could, then alighted on the water, one on the other, till they made a heap as big as the greatest ship, and that the next day, being refreshed by the sun, they took flight again and landed in clouds at Tenerife. De Saussure says[229] that the great oceans are, as a rule, impassable barriers, and that not a species of the tribe Oedipodides has passed from the Old World to the New. It is, however, possible that _Acridium peregrinum_, of the tribe Acridiides, may have originally been an inhabitant of America, and have passed from thence to the Old World.

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The species of Acridiidae that have been ascertained to be migratory are not numerous.[230] The most abundant and widely distributed of them is _Pachytylus cinerascens_ (Fig. 177), which has invaded a large part of the Eastern hemisphere, extending from the Atlantic Ocean to China. It exists in numerous spots in the Oriental region and the Asiatic Archipelago, and even in New Zealand. It is the commoner of the locusts of Europe. Its congener, _P. migratorius_, is much less widely distributed, its migrations being, according to de Saussure, limited to Turkestan and Eastern Europe. A third species, _P. migratorioides_, inhabits Eastern Africa, and a variety of it is the "Yolala" or locust of Madagascar. Mr. Distant has informed the writer that this migratory locust is found in South Africa. _P._ (_Oedaleus_) _marmoratus_ has almost as wide a distribution in the Eastern hemisphere as _P. cinerascens_, except that it is more exclusively tropical; it is thus excluded from New Zealand. _P._ (_Oedaleus_) _nigrofasciatus_ has a more northern distribution than its congener, but has extended to Africa and the Asiatic Archipelago. This Insect is so variable that the distinctions of its races from other species of the same genus are not yet clear. All the above-mentioned locusts belong to the tribe Oedipodides. _Acridium peregrinum_, now more frequently called _Schistocerca peregrina_, belongs to the tribe Acridiides. It is a large locust (Fig. 84), and has a wide distribution. It is the chief species in North Africa, and is probably the locust of the plagues of Egypt mentioned in the book of Exodus. It is also, according to Cotes,[231] the chief locust of North-West India. In this latter country _Pachytylus cinerascens_ and some other species also occur. With the exception of _S. peregrina_, the species of the genus _Schistocerca_ are confined to the New World. In North America locusts are more usually called grasshoppers. Several species of the genus _Caloptenus_ are injurious in that country, but the chief migratory species is _C. spretus_ (Fig. 175). This genus belongs to Acridiides. A large locust, _Schistocerca americana_, is also migratory to a small extent in the United States. In South America other species of _Schistocerca_ are migratory; it is not known how many there may be, and it is possible that one or more may prove to be the _S. peregrina_ of the Old World. A Chilian species, according to Mr. E. C. Reed,[232] {299}exhibits distinctions of colour similar to those that have been observed in _S. peregrina_ in Algeria.

In Britain we are now exempt from the ravages of locusts, though swarms are said to have visited England in 1693 and 1748. Individuals of the migratory species are, however, still occasionally met with in England and the south of Scotland. _P. cinerascens_ has been recorded from Kerry in Ireland, but erroneously, the Insect found being _Mecostethus grossus_ (Fig. 173). According to Miss Ormerod,[233] large locusts are imported to this country in fodder in considerable numbers, but are usually dead; living individuals are, however, sometimes found among the others. In 1869 living specimens of _Schistocerca peregrina_ were found in various parts of the country, having, in all probability, arrived here by crossing the German Ocean. _Pachytylus cinerascens_ has also, it is believed, occurred here, the specimens that have been recorded at different times under the name of _P. migratorius_ being more probably the former species.

Although the majority of the very large number of species included in Acridiidae are recognised with ease from their family likeness as belonging to the group, yet there are others that present an unusual aspect. This is specially the case with the members of the small tribes Tettigides, Proscopides, and Pneumorides, and with some of the apterous forms of the Oedipodides. The tribe Proscopides (Fig. 178, _Cephalocoema lineata_, female) includes some of the most curious of the Acridiidae. Breitenbach gives[234] a brief account of the habits of certain species which he met with near Porto Alegre in South America. On a stony hill there was some grass which, by several months' exposure to the sun's rays, had {300}become withered and brown. Apparently no live thing was to seen on this hillock except the ubiquitous ants, but after a while he noticed some "lightning-like" movements, which he found were due to specimens of Proscopia. The Insects exactly resemble the withered vegetation amongst which they sit, and when alarmed seek safety with a lengthy and most rapid leap. When attention was thus directed to them he found the Insects were really abundant, and was often able to secure fifty specimens on a single afternoon. These Insects bear a great general resemblance to the Phasmides, but there is no evidence at present to show that the two kinds of Insects live in company, as is the case with so many of the Insects that resemble one another in appearance. Although the linear form and the elongation of the body are common to the stick-Insects and the Proscopides, yet this structure is due to the growth of different parts in the two families. In the Phasmidae the prothorax is small, the mesothorax elongate, while in the Proscopides the reverse is the case. The elongation of the head is very curious in these Insects; the mouth is not thus brought any nearer to the front, but is placed on the under side of the head, quite close to the thorax. The tribe Tryxalides contains Insects (Fig. 165) that approach the Proscopides in the form of the head and other characters. In most cases the sexes of the Proscopides differ from one another so strongly that it is difficult to recognise them as being of the same species. Usually both sexes are entirely apterous, but the Chilian genus _Astroma_ exhibits a remarkable exception and an almost unique condition of the alar organs, the mesonotum being in each sex entirely destitute of such appendages, while the female has on the metanotum rudiments of wings which are absent in the male.

The tribe Tettigides is a very extensive group of small Acridiidae, in which the pronotum extends backwards as a hood and covers the body, the tegmina and wings being more or less modified. In our British species (Fig. 179) this condition does not greatly modify the appearance of the Insect, but in many exotic species (Fig. 180) the hood assumes {301}remarkable developments, so that the Insects have no longer the appearance of Orthoptera. It would be impossible, without the aid of many figures, to give an idea of the variety of forms assumed by this prothoracic expansion. It is a repetition of what occurs in the Order Hemiptera, where the prothoracic hoods of the Membracides exhibit a similar, though even more extraordinary, series of monstrous forms. So great is the general similarity of the two groups that when the genus _Xerophyllum_ (Fig. 180, A) was for the first time described, it was treated by the describer as being a bug instead of a grasshopper. This genus includes several species from Africa. The curious _Cladonotus_ (Fig. 180, B) is a native of Ceylon, where it is said to live in sandy meadows, after the fashion of our indigenous species of _Tettix_ (Fig. 179). Very little is known as to the habits of these curious Tettigides, but it has been ascertained that some of the genus _Scelimena_ are amphibious, and do not hesitate to enter the water and swim about there; indeed it is said that they prefer plants growing under water as food. This habit has been observed both in Ceylon and the Himalayas. The species are said to have the hind legs provided with dilated foliaceous appendages useful for swimming.

The tribe Mastacides includes thirty or forty species of Acridiidae with short antennae and vertical head (Fig. 181, _Mastax guttatus_); they are apparently all rare and little {302}known, but are widely distributed in the tropics of the Old and New Worlds. Nothing whatever seems to be known of their habits or of their development.

The tribe Pneumorides includes a still smaller number of species of very aberrant and remarkable grasshoppers, of large size, with short antennae, and with the pronotum prolonged and hood-like; they are peculiar to South Africa. Although amongst the most remarkable of Insects, we are not able to give any information as to their habits. It would appear from the form of their legs that they have but little power of hopping. The species of which we figure the female (Fig. 182) is very remarkable from the difference in colour of the sexes. The female is so extravagantly coloured that she has been said to look as if "got up" for a fancy-dress ball. She is of a gay green, with pearly white marks, each of which is surrounded by an edging of magenta; the white marks are very numerous, especially on the parts of the body not shown in our figure; the face has magenta patches and a large number of tiny pearly-white tubercles, each of which, when placed on a green part, is surrounded by a little ring of mauve colour. Though the female is certainly one of the most remarkably coloured of Insects, her consort is of a modest, almost unadorned green colour, and is considerably different in form. He is, however, provided with a musical apparatus, which it is possible may be a means of pleasing his gorgeous but dumb spouse. It consists of a series of ridges placed on each side of the inflated abdomen, which, as we have previously (p. 200) remarked, has every appearance of being inflated with the result of improving its resonance.

{303}The Pyrgomorphides[235] is a small tribe of about 120 described species, two of which are found in the south of Europe (Fig. 183, _Pyrgomorpha grylloides_). The tribe includes a number of large and curious Insects, among them the species of _Phymateus_ and _Petasia_, with peculiar excrescences on the pronotum and vivid colours on some parts of the body or its appendages, which are apparently common Insects in South Africa.

The tribe Tryxalides includes a great many species of grasshoppers. In them the front of the head joins the upper part at an acute angle (Figs. 165 and 173). This tribe and the Acridiides are the most numerous in species of the family. To the latter belong most of the migratory locusts of the New World (Fig. 175, _Caloptenus spretus_). A Spanish species of this tribe, _Euprepocnemis plorans_, though provided with well-developed wings, possesses the remarkable habit of seeking shelter by jumping into the water and attaching itself below the surface to the stems of plants.

The tribe Pamphagides[236] includes some 200 species, found chiefly in Africa and the arid regions near the Mediterranean Sea. They are mostly apterous forms, and this circumstance has, according to de Saussure, exercised a marked influence on the geographical distribution of the species. Although the tribe consists chiefly of apterous forms, several species possess {304}well-developed wings; sometimes this is the case of the male but not of the female. Some of the species are highly modified for a desert life, and exhibit a great variation in the colour of the individuals in conformity with the tint of the soil they inhabit. _Xiphocera asina_ (Fig. 184) is thought by Péringuey to be the prey of the extraordinary South African tiger-beetles of the genus _Manticora_.

We have already mentioned the tribe Oedipodides[237] as including most of the species of migratory locusts of the Old World. Some striking cases of variation in colour occur amongst the winged Oedipodides. In certain species the hind wings may be either blue or rosaceous in colour; it is thought that the latter is the tint natural in the species, and that it is due to the mixture of a red pigment with the pale blue colour of the wing; hence the blue-coloured wings are analogous to cases of albinism. But the most remarkable fact is that this colour difference is correlative with locality. Brunner von Wattenwyl says[238] that the blue variety of _Oe. variabilis_ occurs only in a few localities in Europe—he mentions Vienna and Sarepta,—and that where it occurs not a single red example can be met with. Similar phenomena occur in other species in both Europe and North America, and L. Bruner has suggested[239] that the phenomena in the latter country are correlative with climatic conditions.

The group Eremobiens, a subdivision of Oedipodides, includes some of the most interesting forms of Acridiidae. Its members have several modes of stridulation. _Cuculligera flexuosa_ and other of the winged forms, according to Pantel,[240] produce sounds by the friction of the middle tibia against the wing, both of these parts being specially modified for the purpose in the male sex. The most peculiar members of the Eremobiens are some very large Insects, modified to an extraordinary extent for a sedentary life in deserts and arid places. Trimen says[241] that a South African species, _Trachypetra bufo_, which lives amongst stones, is so coloured that he had much difficulty in detecting it, and that he noticed in certain spots, often only a few square yards in extent, where the stones lying on the ground were darker, lighter, or more mottled than usual, that the individuals of the grasshopper were of a similar colour to the stones.

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The Insect referred to by Trimen is, we believe, the _Batrachotettix whiti_ of de Saussure. In this species the alar organs are completely absent, and the pronotum forms a sort of hood that protects the base of the hind body. Some of the desert Eremobiens vary so much that the differences found among individuals of the same species {306}are said by Brunner and de Saussure to be so great as to affect even the generic characters, and give rise to the idea of an "uncompleted species-formation."

_Methone anderssoni_, an inhabitant of the Karoo Desert of South Africa, is one of the largest of the Acridiidae. A female of this species is represented of the natural size in Fig. 185. This Insect is remarkable on account of the complex organs for producing sound, and for the great modification of the posterior legs (Fig. 185, _b_), which do not possess locomotive functions, but serve as a portion of the sound-producing apparatus, and as organs for protecting the sides of the body. This Insect is said to be very efficient in making a noise. The sexes differ considerably in their sound-producing organs, a portion of which are present in the female as well as in the male (Fig. 186). Connected with the first abdominal segment, but extending backwards on the second, there is a peculiar swelling bearing two or three strongly raised chitinous folds (Fig. 186, _c_). When the leg is rotated these folds are struck by some peg-like projections situate on the inner face of the base of the femur, and a considerable noise is thus produced. The pegs cannot be seen in our figure. This apparatus is equally well developed in female and male. On the second abdominal segment, immediately behind the creaking folds we have described, there is a prominent area, densely and finely striated (Fig. 186, _d_): this is rubbed by some fine asperities on the inner part of the femur near its base. Sound is produced by this friction on the striated surface, the sculpture of which is abruptly contrasted with that of the contiguous parts: these structures seem to be somewhat better developed in the male than they are in the female, and to be phonetic, at any rate in the former sex. {307}The male has the rudimentary tegmina (Fig. 186, _f_) much longer than they are in the female (Fig. 185), and their prolonged part is deeply grooved, so as to give rise to strong ridges, over which plays the edge of the denticulate and serrate femur. There is nothing to correspond to this in the female, and friction over the surface of this part of the male produces a different and louder sound. There can be little doubt that this is a phonetic structure peculiar to the male. It approximates in situation to the sound-producing apparatus of the males of the Stenobothri and other Acridiidae. _Methone anderssoni_ has large tympanal organs: the small tegmina cover them up completely. In the female the tips of the tegmina seem to be adapted for forming covering-flaps for the tympana. In both sexes there is a sac (Fig. 186, _b_) adjoining the structures we have mentioned, but which is not directly phonetic, though it may be an adjunct of the apparatus.

There is no other Orthopteron in which the phonetic organs are so complex as they are in the male of _Methone anderssoni_, and it would appear probable that this Insect possesses the power of producing two, if not more, distinct sounds, one in common with the female, and peculiar to this and one or two other species; the other somewhat similar to that of other Acridiids, and more specially developed in the male, if not absolutely confined to it.

This Insect is of a very sedentary disposition, and when disturbed apparently seeks safety rather by the noise it can make than by flight. Its powers of locomotion indeed are very feeble. The alar organs are quite rudimentary, and of no assistance whatever for movement. The hind legs seem to be almost equally useless for this purpose; they are broader than they are in other Acridiidae, and have different functions. When _Methone_ moves it does so by means of the anterior four legs, on which it walks propped up as if on stilts. When at rest the hind legs are pressed close to the body, and the tibiae are inflexed and not seen, the creature in this position greatly resembling a clod of earth. We know nothing of the life history of this Insect, except that the young resemble the adult in appearance, and are provided with the sound-producing apparatus, or some portion thereof.