Part 18

The most striking feature in the development of beetles is the great diversity noticeable in the outward form of the larva in different families. The larva of a ground-beetle or a carnivorous water-beetle (fig. 2 c) is an active elongate grub with well-armoured cuticle. The head--carrying feelers, mandibles and two pairs of maxillae--is succeeded by the three thoracic segments, each bearing a pair of strong five-segmented legs, whose feet, like those of the adult, carry two claws. Ten segments can be distinguished in the tapering abdomen, the ninth frequently bearing a pair of tail-feelers (cerci), and the tenth, attached ventrally to the ninth, having the anal opening at its extremity and performing the function of a posterior limb, supporting and temporarily fixing the tail end of the insect on the surface over which it crawls. Such a typically "campodeiform" grub, moving actively about in pursuit of prey, is the one extreme of larval structure to be noticed among the Coleoptera. The other is exemplified by the white, wrinkled, soft-skinned, legless grub of a weevil, which lives underground feeding on roots, or burrows in the tissues of plants (fig. 3 b). Between these two extremes we find various transitional forms: an active larva, as described above, but with four-segmented, single-clawed legs, as among the rove-beetles and their allies; the body well armoured, but slender and worm-like, with very short legs as in wireworms and mealworms (figs. 18, 21 b); the body shortened, with the abdomen swollen, but protected with tubercles and spines, and with longish legs adapted for an active life, as in the predaceous larvae of ladybirds; the body soft-skinned, swollen and caterpillar-like, with legs well developed, but leading a sluggish underground life, as in the grub of a chafer; the body soft-skinned and whitish, and the legs greatly reduced in size, as in the wood-feeding grub of a longhorn beetle. In the case of certain beetles whose larvae do not find themselves amid appropriate food from the moment of hatching, but have to migrate in search of it, an early larval stage, with legs, is followed by later sluggish stages in which legs have disappeared, furnishing examples of what is called hypermetamorphosis. For example, the grub of a pea or bean beetle (_Bruchus_) is hatched, from the egg laid by its mother on the carpel of a leguminous flower, with three pairs of legs and spiny processes on the prothorax. It bores through and enters the developing seed, where it undergoes a moult and becomes legless. Similarly the newly-hatched larva of an oil-beetle (_Meloe_) is an active little campodeiform insect, which, hatched from an egg laid among plants, waits to attach itself to a passing bee. Carried to the bee's nest, it undergoes a moult, and becomes a fat-bodied grub, ready to lead a quiet life feeding on the bee's rich food-stores.

[Illustration: From Chittenden, _Yearbook_, 1894, U.S. Dept. of Agriculture.

FIG. 3.--Grain Weevils. a, _Calandra granaria_; b, larva; c, pupa; d, _C. oryzae_.]

_Distribution and Habits._--The Coleoptera are almost world-wide in their distribution, being represented in the Arctic regions and on almost all oceanic islands. Most of the dominant families--such as the _Carabidae_ (ground-beetles), _Scarabaeidae_ (chafers), or _Curculionidae_ (weevils) have a distribution as wide as the order. But while some large families, such as the _Staphylinidae_ (rove-beetles) are especially abundant on the great northern continents, becoming scarcer in the tropics, others, the _Cicindelidae_ (tiger-beetles), for example, are most strongly represented in the warmer regions of the earth, and become scarce as the collector journeys far to south or north. The distribution of many groups of beetles is restricted in correspondence with their habits; the _Cerambycidae_ (longhorns), whose larvae are wood-borers, are absent from timberless regions, and most abundant in the great tropical forests. Some families are very restricted in their range. The _Amphizoidae_, for example, a small family of aquatic beetles, are known only from western North America and Eastern Tibet, while an allied family, the _Pelobiidae_, inhabit the British Isles, the Mediterranean region, Tibet and Australia. The beetles of the British islands afford some very interesting examples of restricted distribution among species. For example, large and conspicuous European beetles, such as the stag-beetle (fig. 1, _Lucanus cervus_) and the great water-beetle (_Hydrophilus piceus_, fig. 20), are confined to eastern and southern Britain, and are unknown in Ireland. On the other hand, there are Arctic species like the ground-beetle, _Pelophila borealis_, and south-western species like the boring weevil, _Mesites Tardyi_, common in Ireland, and represented in northern or western Britain, but unknown in eastern Britain or in Central Europe. Careful study of insular faunas, such as that of Madeira by T. V. Wollaston, and of the Sandwich Islands by D. Sharp, and the comparison of the species found with those of the nearest continental land, furnish the student of geographical distribution with many valuable and suggestive facts.

Notes on habit are given below in the accounts of the various families. In general it may be stated that beetles live and feed in almost all the diverse ways possible for insects. There are carnivores, herbivores and scavengers among them. Various species among those that are predaceous attack smaller insects, hunt in packs crustaceans larger than themselves, insert their narrow heads into snail-shells to pick out and devour the occupants, or pursue slugs and earthworms underground. The vegetable-feeders attack leaves, herbaceous or woody stems and roots; frequently different parts of a plant are attacked in the two active stages of the life-history; the cockchafers, for example, eating leaves, and their grubs gnawing roots. Some of the scavengers, like the burying beetles, inter the bodies of small vertebrates to supply food for themselves and their larvae, or, like the "sacred" beetle of Egypt, collect for the same purpose stores of dung. Many beetles of different families have become the "unbidden guests" of civilized man, and may be found in dwelling-houses, stores and ships' cargoes, eating food-stuffs, paper, furniture, tobacco and drugs. Hence we find that beetles of some kind can hold their own anywhere on the earth's surface. Some climb trees and feed on leaves, while others tunnel between bark and wood. Some fly through the air, others burrow in the earth, while several families have become fully adapted to life in fresh water. A large number of beetles inhabit the deep limestone caves of Europe and North America, while many genera and some whole families are at home nowhere but in ants' nests. Most remarkable is the presence of a number of beetles along the seashore between tide-marks, where, sheltered in some secure nook, they undergo immersion twice daily, and have their active life confined to the few hours of the low ebb.

_Stridulating Organs._--Many beetles make a hissing or chirping sound by rubbing a "scraper," formed by a sharp edge or prominence on some part of their exoskeleton, over a "file" formed by a number of fine ridges situate on an adjacent region. These stridulating organs were mentioned by C. Darwin as probable examples of the action of sexual selection; they are, however, frequently present in both sexes, and in some families also in the larvae. An account of the principal types of stridulators that have been described has been published by C. J. Gahan (1900). The file may be on the head--either upper or lower surface--and the scraper formed by the front edge of the prothorax, as in various wood-boring beetles (_Anobium_ and _Scolytus_). Or ridged areas on the sides of the prothorax may be scraped by "files" on the front thighs, as in some ground-beetles. Among the longhorn beetles, the prothorax scrapes over a median file on the mid-dorsal aspect of the mesothorax. In a large number of beetles of different families, stridulating areas occur on various segments of the abdomen, and are scraped by the elytra. It is remarkable that these organs are found in similar positions in genera belonging to widely divergent families, while two genera of the same family may have them in different positions. It follows, therefore, that they have been independently acquired in the course of the evolution of the Coleoptera.

Stridulating organs among beetle-larvae have been noted, especially in the wood-feeding grub of the stag-beetles (_Lucanidae_) and their allies the _Passalidae_, and in the dung-eating grubs of the dor-beetles (_Geotrupes_), which belong to the chafer family (_Scarabaeidae_). These organs are described by J. C. Schiödte and D. Sharp; in the stag-beetle larva a series of short tubercles on the hind-leg is drawn across the serrate edge of a plate on the haunch of the intermediate legs, while in the Passalid grub the modified tip of the hind-leg acts as a scraper, being so shortened that it is useless for locomotion, but highly specialized for producing sound. Whatever may be the true explanation of stridulating organs in adult beetles, sexual selection can have had nothing to do with the presence of these highly-developed larval structures. It has been suggested that the power of stridulation would be advantageous to wood-boring grubs, the sound warning each of the position of its neighbour, so that adjacent burrowers may not get in each other's way. The root-feeding larvae of the cockchafer and allied members of the _Scarabaeidae_ have a ridged area on the mandible, which is scraped by teeth on the maxillae, apparently forming a stridulating organ.

_Luminous Organs._--The function of the stridulating organs just described is presumably to afford means of recognition by sound. Some beetles emit a bright light from a portion of their bodies, which leads to the recognition of mate or comrade by sight. In the wingless female glow-worm (_Lampyris_, fig. 15 f) the luminous region is at the hinder end, the organ emitting the light consisting, according to H. von Wielowiejski (1882), of cells similar to those of the fat-body, containing a substance that undergoes oxidation. The illumination is intermittent, and appears to be under the control of the insect's nervous system. The well-known "fire-flies" of the tropics are large click-beetles (_Elateridae_), that emit light from paired spots on the prothorax and from the base of the ventral abdominal region. The luminous organs of these beetles consist of a specialized part of the fat-body, with an inner opaque and an outer transparent layer. Its structure has been described by C. Heinemann, and its physiology by R. Dubois (1886), who considers that the luminosity is due to the influence of an enzyme in the cells of the organ upon a special substance in the blood. The eggs and larvae of the fire-flies are luminous as well as the perfect beetles.

_Fossil History._--The Coleoptera can be traced back farther in time than any other order of insects with complete transformations, if the structures that have been described from the Carboniferous rocks of Germany are really elytra. In the Triassic rocks of Switzerland remains of weevils (_Curculionidae_) occur, a family which is considered by many students the most specialized of the order. And when we know that the _Chrysomelidae_ and _Buprestidae_ also lived in Triassic, and the _Carabidae_, _Elateridae_, _Cerambycidae_ and _Scarabaeidae_, in Liassic times, we cannot doubt that the great majority of our existing families had already been differentiated at the beginning of the Mesozoic epoch. Coming to the Tertiary we find the Oligocene beds of Aix, of east Prussia (amber) and of Colorado, and the Miocene of Bavaria, especially rich in remains of beetles, most of which can be referred to existing genera.

_Classification._--The Coleoptera have been probably more assiduously studied by systematic naturalists than any other order of insects. The number of described species can now hardly be less than 100,000, but there is little agreement as to the main principles of a natural classification. About eighty-five families are generally recognized; the difficulty that confronts the zoologists is the arrangement of these families in "superfamilies" or "sub-orders." Such obvious features as the number of segments in the foot and the shape of the feeler were used by the early entomologists for distinguishing the great groups of beetles. The arrangement dependent on the number of tarsal segments--the order being divided into tribes _Pentamera_, _Tetramera_, _Heteromera_ and _Trimera_--was suggested by E. L. Geoffroy in 1762, adopted by P. A. Latreille, and used largely through the 19th century. W. S. Macleay's classification (1825), which rested principally on the characters of the larvae, is almost forgotten nowadays, but it is certain that in any systematic arrangement which claims to be natural the early stages in the life-history must receive due attention. In recent years classifications in part agreeing with the older schemes but largely original, in accord with researches on the comparative anatomy of the insects, have been put forward. Among the more conservative of these may be mentioned that of D. Sharp (1899), who divides the order into six great series of families: _Lamellicornia_ (including the chafers and stag-beetles and their allies with five-segmented feet and plate-like terminal segments to the feelers); _Adephaga_ (carnivorous, terrestrial and aquatic beetles, all with five foot-segments); _Polymorpha_ (including a heterogeneous assembly of families that cannot be fitted into any of the other groups); _Heteromera_ (beetles with the fore and intermediate feet five-segmented, and the hind-feet four-segmented); _Phytophaga_ (including the leaf-beetles, and longhorns, distinguished by the apparently four-segmented feet), and _Rhynchophora_ (the weevils and their allies, with head prolonged into a snout, and feet with four segments). L. Ganglbauer (1892) divides the whole order into two sub-orders only, the _Caraboidea_ (the _Adephaga_ of Sharp and the older writers) and the _Cantharidoidea_ (including all other beetles), since the larvae of _Caraboidea_ have five-segmented, two-clawed legs, while those of all other beetles have legs with four segments and a single claw. A. Lameere (1900) has suggested three sub-orders, the _Cantharidiformia_ (including the _Phytophaga_, the _Heteromera_, the _Rhynchophora_ and most of the _Polymorpha_ of Sharp's classification), the _Staphyliniformia_ (including the rove-beetles, carrion-beetles and a few allied families of Sharp's _Polymorpha_), and the _Carabidiformia_ (_Adephaga_). Lameere's classification is founded on the number of abdominal sterna, the nervuration of the wings, the number of malpighian tubules (whether four or six) and other structural characters. Preferable to Lameere's system, because founded on a wider range of adult characters and taking the larval stages into account, is that of H. J. Kolbe (1901), who recognizes three sub-orders: (i.) the _Adephaga_; (ii.) the _Heterophaga_, including the _Staphylinoidea_, the _Actinorhabda_ (_Lamellicornia_), the _Heterorhabda_ (most of Sharp's _Polymorpha_), and the _Anchistopoda_ (the _Phytophaga_, with the ladybirds and some allied families which Sharp places among the _Polymorpha_); (iii.) the _Rhynchophora_.

Students of the Coleoptera have failed to agree not only on a system of classification, but on the relative specialization of some of the groups which they all recognize as natural. Lameere, for example, considers some of his _Cantharidiformia_ as the most primitive Coleoptera. J. L. Leconte and G. H. Horn placed the _Rhynchophora_ (weevils) in a group distinct from all other beetles, on account of their supposed primitive nature. Kolbe, on the other hand, insists that the weevils are the most modified of all beetles, being highly specialized as regards their adult structure, and developing from legless maggots exceedingly different from the adult; he regards the Adephaga, with their active armoured larvae with two foot-claws, as the most primitive group of beetles, and there can be little doubt that the likeness between larvae and adult may safely be accepted as a primitive character among insects. In the Coleoptera we have to do with an ancient yet dominant order, in which there is hardly a family that does not show specialization in some point of structure or life-history. Hence it is impossible to form a satisfactory linear series.

In the classification adopted in this article, the attempt has been made to combine the best points in old and recent schemes, and to avoid the inconvenience of a large heterogeneous group including the vast majority of the families.

ADEPHAGA.--This tribe includes beetles of carnivorous habit with five segments on every foot, simple thread-like feelers with none of the segments enlarged to form club or pectination, and the outer lobs (galea) of the first maxilla usually two-segmented and palpiform (fig. 4 b). The transverse fold of the hind-wing is towards the tip, about two-thirds of the wing-length from the base. At this fold the median nervure stops and is joined by a cross nervure to the radial, which can be distinguished throughout its length from the subcostal. There are four malpighian tubules. In the ovarian tubes of Adephaga small yolk-chambers alternate with the egg-chambers, while in all other beetles there is only a single large yolk-chamber at the narrow end of the tube. The larvae (fig. 2 c) are active, with well-chitinized cuticle, often with elongate tail-feelers (cerci), and with five-segmented legs, the foot-segment carrying two claws.

[Illustration: FIG. 4.--_Mormolyce phyllodes_. Java. a, Labium; b, maxilla; c, labrum; d, mandible.]

The generalized arrangement of the wing-nervure and the nature of the larva, which is less unlike the adult than in other beetles, distinguish this tribe as primitive, although the perfect insects are, in the more dominant families, distinctly specialized. Two very small families of aquatic beetles seem to stand at the base of the series, the _Amphizoidae_, whose larvae are broad and well armoured with short cerci, and the _Pelobiidae_, which have elongate larvae, tapering to the tail end, where are long paired cerci and a median process, recalling the grub of a Mayfly.

[Illustration: FIG. 5.--_Pheropsophus Jurinei_. W. Africa.]

[Illustration: FIG. 6.--_Carabus rutilans_. Spain.]

The _Dyticidae_ (fig. 2) are Adephaga highly specialized for life in the water, the hind-legs having the segments short, broad and fringed, so as to be well adapted for swimming, and the feet without claws. The metasternum is without the transverse linear impression that is found in most families of Adephaga. The beetles are ovoid in shape, with smooth contours, and the elytra fit over the edges of the abdomen so as to enclose a supply of air, available for use when the insect remains under water. The fore-legs of many male dyticids have the three proximal foot-segments broad and saucer-shaped, and covered with suckers, by means of which they secure a firm hold of their mates. Larval dyticids (fig. 2 b) possess slender, curved, hollow mandibles, which are perforated at the tip and at the base, being thus adapted for sucking the juices of victims. Large dyticid larvae often attack small fishes and tadpoles. They breathe by piercing the surface film with the tail, where a pair of spiracles are situated. The pupal stage is passed in an earthen cell, just beneath the surface of the ground. Nearly 2000 species of _Dyticidae_ are known: they are universally distributed, but are most abundant in cool countries. The _Haliplidae_ form a small aquatic family allied to the _Dyticidae_.

[Illustration: FIG. 7.--_Cicindela sylvatica_ (Wood Tiger-Beetle). Europe.]

[Illustration: FIG. 8.--_Manticora tuberculata_. S. Africa.]

The _Carabidae_, or ground-beetles, comprising 13,000 species, form the largest and most typical family of the Adephaga (figs. 4, 5, 6), the legs of all three pairs being alike and adapted for rapid running. In many _Carabidae_ the hind-wings are reduced or absent, and the elytra fused together along the suture. Many of our native species spend the day lurking beneath stones, and sally forth at night in pursuit of their prey, which consists of small insects, earthworms and snails. But a number of the more brightly coloured ground-beetles run

## actively in the sunshine. The carabid larva is an active well-armoured

grub with the legs and cerci variable in length. Great differences in the general form of the body may be observed in the family. For example, the stout, heavy body of _Carabus_ (fig. 6) contrasts markedly with the wonderful flattened abdomen and elytra of _Mormolyce_ (fig. 4), a Malayan genus found beneath fallen trees, a situation for which its compressed shape is admirably adapted. Blind _Carabidae_ form a large proportion of cave-dwelling beetles, and several species of great interest live between tide-marks along the seashore.

The _Cicindelidae_, or tiger-beetles (figs. 7, 8) are the most highly organized of all the Adephaga. The inner lobe (lacinia) of the first maxilla terminates in an articulated hook, while in the second maxillae (labium) both inner and outer lobes ("ligula" and "para-glossae") are much reduced. The face (clypeus) is broad, extending on either side in front of the insertion of the feelers. The beetles are elegant insects with long, slender legs, running quickly, and flying in the sunshine. The pronotum and elytra are often adorned with bright colours or metallic lustre, and marked with stripes or spots. The beetles are fierce in nature and predaceous in habit, their sharp toothed mandibles being well adapted for the capture of small insect-victims. The larvae are more specialized than those of other Adephaga, the head and prothorax being very large and broad, the succeeding segments slender and incompletely chitinized. The fifth abdominal segment has a pair of strong dorsal hook-like processes, by means of which the larva supports itself in the burrow which it excavates in the earth, the great head blocking the entrance with the mandibles ready to seize on any unwary insect that may venture within reach.

[Illustration: FIG. 9.

a _Gyrinus sulcatus_ (Grooved Whirligig). Europe. b Antenna of _Gyrinus_. c Larva of _Gyrinus_.]

Two or three families may be regarded as aberrant Adephaga. The _Paussidae_ are a very remarkable family of small beetles, mostly tropical, found only in ants' nests, or flying by night, and apparently migrating from one nest to another. The number of antennal segments varies from eleven to two. It is supposed that these beetles secrete a sweet substance on which the ants feed, but they have been seen to devour the ants' eggs and grubs. The _Gyrinidae_, or whirligig beetles (fig. 9), are a curious aquatic family with the feelers (fig. 9, b) short and reduced as in most _Paussidae_. They are flattened oval in form, circling with gliding motion over the surface film of the water, and occasionally diving, when they carry down with them a bubble of air. The fore-legs are elongate and adapted for clasping, while the short and flattened intermediate and hind legs form very perfect oar-like propellers. The larva of _Gyrinus_ (fig. 9, c) is slender with elongate legs, and the abdominal segments carry paired tracheal gills.

STAPHYLINOIDEA.--The members of this tribe may be easily recognized by their wing-nervuration. Close to a transverse fold near the base of the wing, the median nervure divides into branches which extend to the wing-margin; there is a second transverse fold near the tip of the wing, and cross nervures are altogether wanting. There are four malpighian tubes, and all five tarsal segments are usually recognizable. With very few exceptions, the larva in this group is

## active and campodeiform, with cerci and elongate legs as in the

Adephaga, but the leg has only four segments and one claw.

[Illustration: FIG. 10.--_Silpha quadripunctata_. Europe.]

[Illustration: FIG. 11.--_Necrophorus vespillo_ (Sexton Beetle). Europe.]