Hexapoda Sentence Examples

For the relationship of the Coleoptera to other orders of insects see Hexapoda.

A true insect, or member of the class Hexapoda, may be known by the grouping of its body-segments in three distinct regions - a head, a thorax and an abdomen - each of which consists of a definite number of segments.

The compound eyes of insects resemble so closely the similar organs in Crustaceans that there can hardly be reasonable doubt of their homology, and the primitively appendicular nature of the eyes in the latter class suggests that in the Hexapoda also they represent the appendages of an anterior (protocerebral) segment.

From these three pairs of thoracic legs comes the name - Hexapoda - which distinguishes the class.

And the wings, though not always present, are highly characteristic of the Hexapoda, since no other group of the Arthropoda has acquired the power of flight.

In all main points of their internal structure the Hexapoda agree with other Arthropoda.

The possession of a variable number of excretory tubes (Malpighian tubes), which are developed as outgrowths of the hind-gut and pour their excretion into the intestine,is also a distinctive character of the Hexapoda.

With this brief summary of the essential characters of the Hexapoda, we may pass to a more detailed account of their structure.

As mentioned above, respiration by means of airtubes (tracheae) is a most characteristic feature of the Hexapoda.

All the Hexapoda are of separate sexes.

In various groups of the Hexapoda - aphids and some flesh-flies (Sarcophagi), for example - the egg undergoes development within the body of the mother, and the young insect is born in an active state; such insects are said to be " viviparous."

On the other hand, we find in the vast majority of the Hexapoda a very marked difference between the perfect insect (imago) and the young animal when newly hatched and for some time after hatching.

Sharp (1898), the marked divergence among the Hexapoda, as regards life-history, is between insects whose wings develop outside the cuticle (Exopterygota) and those whose wings develop inside the cuticle (Endopterygota), becoming visible only when the casting of the last larval cuticle reveals the pupa.

But in general we find that elaboration of imaginal structure is associated with degradation in the nature of the larva, cruciform and vermiform larvae being characteristic of the highest orders of the Hexapoda, so that unlikeness between parent and offspring has increased with the evolution of the class.

In conjunction with the association mentioned above of the most highly developed imaginal with the most degraded larval structure, it indicates clearly that the active, armoured grub preceded the sluggish soft-skinned caterpillar or maggot in the evolution of the Hexapoda.

The term nymph is applied by many writers on the Hexapoda to all young forms of insects that are not sufficiently unlike their parents to be called larvae.

Among the Hexapoda generally there is no subsequent ecdysis nor any further growth after the assumption of the winged state.

In the metabolic Hexapoda the resting pupal instar shows externally the wings and other characteristic imaginal organs which have been gradually elaborated beneath the larval cuticle.

The division of the winged Hexapoda into Exopteryga and Endopteryga is thus again justified.

In the present article it is only possible to treat of the division of the Hexapoda into orders and sub-orders and of the relations of these orders to each other.

His order of wingless insects (Aptera) included Crustacea, spiders, centipedes and other creatures that now form classes of the Arthropoda distinct from the Hexapoda; it also included Hexapoda of parasitic and evidently degraded structure, that are now regarded as allied more or less closely to various winged insects.

Consequently the modern order Aptera comprises only a very small proportion of Linnaeus's " Aptera " - the spring-tails and bristle-tails, wingless Hexapoda that stand evidently at a lower grade of development than the bulk of the class.

The great advance in modern zoology as regards the classification of the Hexapoda lies in the treatment of a heterogeneous assembly which formed Linnaeus's order Neuroptera.

Brauer (1885), who separated the spring tails and bristle-tails as a sub-class Apterygogenea from all the other Hexapoda, these forming the sub-class Pterygogenea distributed into sixteen orders.

Handlirsch (1903-1904) - the Hexapoda are divided into four classes and thirty-four orders!

Such excessive multiplication of the larger taxonomic divisions shows an imperfect sense of proportion, for if the term " class " be allowed its usual zoological value, no student can fail to recognize that the Hexapoda form a single welldefined class, from which few entomologists would wish to exclude even the Apterygogenea.

Hexapoda mostly with wings, the wingless forms clearly degraded.

Hexapoda mostly with wings; the wingless forms clearly degraded or modified.

The Hexapoda, being aerial, terrestrial and fresh-water animals, are but occasionally preserved in stratified rocks, and our knowledge of extinct members of the class is therefore fragmentary, while the description, as insects, of various obscure fossils, which are perhaps not even Arthropods, has not tended to the advancement of this branch of zoology.

The latter has established, for all the Palaeozoic insects, an order Palaeodictyoptera, there being a closer similarity between the fore-wings and the hind-wings than is to be seen in most living orders of Hexapoda, while affinities are shown to several of these orders - notably the Orthoptera, Ephemeroptera, Odonata and Hemiptera.

Further details of the geological history of the Hexapoda will be found in the special articles on the various orders.

Geographical Distribution The class Hexapoda has a world-wide range, and so have most of its component orders.

The study of the Hexapoda of oceanic islands is full of interest.

Relationships And Phylogeny The Hexapoda form a very clearly defined class of the Arthropoda, and many recent writers have suggested that they must have arisen independently of other Arthropods from annelid worms, and that the Arthropoda must, therefore, be regarded as an " unnatural," polyphyletic assemblage.

It is admitted on all hands that the Hexapoda are akin to the Chilopoda.

Carpenter (1899, 1902-1904) has lately endeavoured to show an exact numerical correspondence in segmentation between the Hexapoda, the Crustacea, the Arachnida, and the most primitive of the Diplopoda.

On either view it may be believed that the Hexapoda arose with the allied classes from a primitive arthropod stock, while the relationships of the class are with the Crustacea, the Chilopoda and the Diplopoda, rather than with the Arachnida.

Some of those zoologists who look to Peripatus, or a similar worm-like form, as representing the direct ancestors of the Hexapoda have laid stress on a larva like the caterpillar of a moth or saw-fly as representing a primitive stage.

On the other hand, it has been argued that the presence of wings in a vast majority of the Hexapoda suggests their presence in the ancestors of the whole class.

It is most unlikely that wings have been acquired independently by various orders of Hexapoda, and if we regard the Thysanura as the slightly modified representatives of a primitively wingless stock, we must postulate the acquisition of wings by some early offshoot of that stock, an offshoot whence the whole group of the Pterygota took its rise.

But a survey of the Hexapoda as a whole, and especially a comparative study of the tracheal system, can hardly leave room for doubt that this system is primitively adapted for atmospheric breathing, and that the presence of tracheal gills in larvae must be regarded as a special adaptation for temporary aquatic life.

The sub-imago of the Ephemeroptera suggests that a moult, after the wings had become functional, was at one time general among the Hexapoda, and that the resting nymph of the Thysanoptera or the pupa of the Endopterygota represents a formerly active stage in the life-history.

So widely have most of the higher orders of the Hexapoda now diverged from each other, that it is exceedingly difficult in most cases to trace their relationships with any confidence.

The eruciform larva of the Orthorrhapha leads on to the headless vermiform maggot of the Cyclorrhapha, and in the latter sub-order we find metamorphosis carried to its extreme point, the muscid flies being the most highly specialized of all the Hexapoda as regards structure, while their maggots are the most degraded of all insect larvae.

They differ from other Endopterygota in the multiplication of their Malpighian tubes, and from all other Hexapoda in the union of the first abdominal segment with the thorax.

In some way it is assured among the highest of the Hexapoda - the Lepidoptera, Diptera and Hymenoptera - that the larva finds itself amid a rich food-supply.

The fascinating difficulties presented to the student by the metamorphosis of the Hexapoda are to some extent explained, as he ponders over the evolution of the class.

References to the older classical writings on the Hexapoda are given in the article on Entomology.

At present about a thousand works and papers are published annually, and in this place it is possible to enumerate only a few of the most important among (mostly) recent memoirs that bear upon the Hexapoda generally.

References to the works of the above authors, and to many others, will be found under HEXAPODA and the special articles on various insect orders.

For these Lamarck proposed the name Hexapoda; but that name has been little used, and they have retained to this day the title of the much larger Linnaean group, viz.

The Arachnida form a distinct class or line of descent in the grade Euarthropoda, diverging (perhaps in common at the start with the Crustacea) from primitive Euarthropods, which gave rise also to the separate lines of descent known as the classes Diplopoda, Crustacea, Chilopoda and Hexapoda.

Watase has shown, in a very convincing way, how by deepening the pit-like set of cells beneath a simple lens the more complex ommatidia of the compound eyes of Crustacea and Hexapoda may be derived from such a condition as that presented in the lateral eyes of Limulus and Scorpio.

In other words, the Malpighian tubes of the terrestrial Arachnida are homoplastic with those of Hexapoda and Myriapoda, and not homogenetic with them.

The relationship of the Hymenoptera to other orders of insects is discussed in the article Hexapoda, but it may be mentioned here that in structure the highest members of the order are remarkably specialized, and that in the perfection of their instincts they stand at the head of all insects and indeed of all invertebrate animals.

Needham the wing-neuration in this order is the most primitive to be found in the Hexapoda.

A curious feature among them is the frequent reduction of the wings in the males of certain species, contrary to the usual condition among the Hexapoda, where if the sexes differ in the development of their wings it is the female which has them reduced.

The tenth abdominal segment carries strong, unjointed cerci, while the presence of reproductive armature on the second abdominal segment of the male is a character found in no other order of the Hexapoda.

These families are described and their affinities discussed in the articles Neuroptera and Hexapoda (qq.v.).

Information as to the internal structure of a typical orthopteron - the cockroach - will be found under Hexapoda.

They are remarkable for the possession of complex ears (described in the article Hexapoda) anti From Carpenter's Insects.

The Orthoptera have been largely used for anatomical and embryological researches, the more important of which are mentioned under Hexapoda (q.v.).

But the bristle-tails and springtails, which form the modern order Aptera, are all without any trace of wings, and, on account of several remarkable archaic characters which they exhibit, there is reason for believing that they are primitively wingless - that they represent an early offshoot which sprang from the ancestral stock of the Hexapoda before organs of flight had been acquired by the class.

Aristotle had included in one class "Entoma" the six-legged arthropods which form the modern zoological class of the Hexapoda or Insecta, besides the Arachnida, the centipedes and the millipedes.

For a general account of the structure, development and relationships of insects, see Arthropoda and Hexapoda, while details of the form, habits and classification of insects will be found in articles on the various orders or groups of orders (Aptera, Coleoptera, Dipteria,Hemiptera,Hymenoptera,Lepidoptera,Neuroptera, Orthoptera, Thysanoptera), and in special articles on the more familiar divisions (ANT, BEE, Dragon-Fly, Earwig, &c.).

The names Condylopoda and Gnathopoda have been subsequently proposed for the same group. The word refers to the jointing of the chitinized exo-skeleton of the limbs or lateral appendages of the animals included, which are, roughly speaking, the Crustacea, Arachnida, Hexapoda (so-called " true insects "), Centipedes and Millipedes.

Lamarck's penetrating genius is chiefly responsible for the shrinkage of the word Insecta, since it was he who, forty years after Linnaeus's death, set up and named the two great classes Crustacea and Arachnida (included by Linnaeus under Insecta as the order " Aptera "), assigning to them equal rank with the remaining Insecta of Linnaeus, for which he proposed the very appropriate class-name " Hexapoda."

Lamarck, however, appears not to have insisted on this name Hexapoda, and so the class of Pterygote Hexapods came to retain the group-name Insecta, which is, historically or etymologically, no more appropriate to them than it is to the classes Crustacea and Arachnida.

The existence of this third prosthomere corresponding to the third prosthomere of the Crustacea is a strong argument for the derivation of the Hexapoda, and with them the Chilopoda, from some offshoot of the Crustacean stem or class.

The Hexapoda are not known to us in their earlier or more primitive manifestations; we only know them as possessed of a definite number of somites arranged in definite numbers in three great tagmata.

It is not true that all the biting processes of the Arthropod limb are thus produced - for instance, the jaws of Peripatus are formed by the axis or corm itself, whilst the poisonjaws of Chilopods, as also their maxillae, appear to be formed rather by the apex or terminal region of the ramus of the limb; but the opposing jaws (= hemignaths) of Crustacea, Arachnida and Hexapoda are gnathobases, and not the axis or corm.

In Crustacea and Hexapoda of all grades we find compound eyes with the more complicated ommatidia described above.

But they seem to point to a community of origin of Hexapods and Crustacea in regard to the complicated ommatidia of the compound eye, and to a certain isolation of the Arachnida, which are, however, traceable, so far as the eyes are concerned, to a distant common origin with Crustacea and Hexapoda through the very simple compound eyes (monostichous, polymeniscous) of Limulus.

The lateral eyes of Crustacea are polymeniscous, with highly specialized retinulae like those of Hexapoda, and unlike the simpler compound lateral eyes of lower Arachnida.

It appears from observation of the embryo that whilst the first prosthomere of Centipedes has its appendages reduced and represented only by eye-patches (as in Arachnida, Crustacea and Hexapoda), the second has a rudimentary antenna, which disappears, whilst the third carries the permanent antennae, which accordingly correspond to the second antennae of Crustacea, and are absent in Hexapoda.

The Hexapoda are not only all confined to a very definite disposition of the somites, appendages and apertures, as thus indicated, but in other characters also they present the specialization of a narrowly-limited highly-developed order of such a class as the Crustacea rather than a range from lower more generalized to higher more specialized forms such as that group and also the Arachnida present.

It seems to be a legitimate conclusion that the most primitive Hexapoda were provided with wings, and that the term Pterygota might be used as a synonym of Hexapoda.

Many Hexapoda have lost either one pair or both pairs of wings; cases are common of wingless genera allied to ordinary Pterygote genera.

The wings of Hexapoda are lateral expansions of the terga of the second and third thoracic somites.

The Hexapoda are all provided with a highly developed tracheal system, which presents considerable variation in regard to its stigmata or orifices of communication with the exterior.

When the facile tendency of Arthropoda to develop tracheal air-tubes is admitted, it becomes probable that the tracheae of Hexapods do not all belong to one original system, but may be accounted for by new developments within the group. Whether the primitive tracheal system of Hexapoda was a closed one or open by serial stigmata in every somite remains at present doubtful, but the intimate relation of the system to the wings and tracheal gills cannot be overlooked.

The lateral eyes of Hexapoda, like those of Crustacea, belong to the most specialized type of " compound eye," found only in these two classes.

We can gather no indication of the forefathers of the Hexapoda or of the Chilopoda less specialized than they are, whilst possessing the essential characteristics of these classes.

On the other hand, the facts that the Hexapoda and the Chilopoda have triprosthomerous heads, that the Hexapoda have the same total number of somites as the nomomeristic Crustacea, and the same number of opisthomeres in the head as the more terrestrial Crustacea, together with the same adaptation of the form of important appendages in corresponding somites, and that the compound eyes of both Crustacea and Hexapoda are extremely specialized and elaborate in structure and identical in that structure, all lead to the suggestion that the Hexapoda, and with them, at no distant point, the Chilopoda, have branched off from the Crustacean main stem as specialized terrestrial lines of descent.

And it seems probable that in the case of the Hexapoda, at any rate, the point of departure was subsequent to the attainment of the nomomeristic character presented by the higher grade of Crustacea.

Its condition thus recalls the pupal instar of the higher (Endopterygote) Hexapoda; and the Thysanoptera, though few in number, are seen to be of great interest to the student, exhibiting at once a transition between the biting and the suctorial mouth, and the passage from " incomplete " to " complete " metamorphosis.

This flexible tube is the functional ovipositor, the typical insectan ovipositor with its three pairs of processes (see Hexapoda) being undeveloped among the Coleoptera.

The embryonic development (see Hexapoda) has been carefully studied in several genera of beetles.

On this view Wheeler, however, compares with the " dorsal organ " the peculiar the entire food-canal in most Hexapoda must be regarded as of extra embryonic membrane or indusium which he has observed ectodermal origin, the " endoblast " represents mesoderm only, between serosa and amnion in the embryo of the grasshopper and the median furrow whence it arises can be no longer compared Xiphidium.

Metamorphosis among the Hexapoda depends upon the universal acquisition of wings After Howard, Insect Life, vol.

Like most ametabolic and hemimetabolic Hexapoda, such nymphs continue to move and feed throughout their lives.

These three authors definitely separated the Arachnida, Crustacea and Myriapoda as classes distinct from the Insecta (see Hexapoda).

Altogether, eight orders are recognized in the classification adopted here, the first five of these belonging to the sub-class Exopterygota and the last three to the Endopterygota (see Hexapoda).

The embryonic development of several genera of Aptera, which has been carefully studied, will be more suitably described in comparison with that of other insects than here (see Hexapoda).

The compound eyes are very similar in the details of their structure (see Arthropoda) to those of insects (Hexapoda).