In their life-history the Thysanoptera belong to the Exopterygote division of the Hexapoda (q.v.).
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.
COLEOPTERA, a term used in zoological classification for the true beetles which form one of the best-marked and most natural of the orders into which the class Hexapoda (or Insecta) has been divided.
In addition to what may be found in numerous important works on the Hexapoda (q.v.) as a whole, such as J.
DIPTERA (Sis, double, 7rTEpa, wings), a term (first employed in its modern sense by Linnaeus, Fauna Suecica, 1st ed., 1746, p. 306) used in zoological classification for one of the Orders into which the Hexapoda, or Insecta, are divided.
Brisson (1756), is still adopted by many zoologists, while others prefer the name Hexapoda, first used systematically in its modern sense by P. A.
Latreille in 1825 (Families naturelles du regne animal), since it has the advantage of expressing, in a single word, an important characteristic of the group. The terms "Hexapoda " and " hexapod " had already been used by F.
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.
With this brief summary of the essential characters of the Hexapoda, we may pass to a more detailed account of their structure.
- A striking feature in the food-canal of the Hexapoda, as in other Arthropods, is the great extent of the " foregut " and " hind-gut," lined with a chitinous cuticle, continuous with the exoskeleton.
All the Hexapoda are of separate sexes.
- Among the Hexapoda, as in Arthropods generally, the egg is large, containing an accumulation of yolk for the nourishment of the growing embryo.
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."
- A number of cases are known among the Hexapoda of the development of young from the eggs of virgin females.
Polar bodies were first observed in the eggs of Hexapoda by F.
Part by the yolk, and in part by those anterior and posterior rudiments Its composition has been the subject of prolonged difference of which usually form the mesenteron, but that in some Hexapoda opinion.
Development of most Hexapoda is the formation of a protective These six divisions of the head are diverse in kind, and subsequently undergo so much change that the part each of them takes in the formation of the head-capsule is not finally determined.
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.
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.
Like most ametabolic and hemimetabolic Hexapoda, such nymphs continue to move and feed throughout their lives.
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.
Hence the grouping of the orders of winged Hexapoda into the divisions Exopterygota and Endopterygota, as suggested by D.
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.