Watase, " On the Morphology of the Compound Eyes of Arthropods," Studies from the Biol.
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.
The connexion is not so intimate in Scorpio, but is nevertheless a very close one, closer than we find in any other Arthropods in which the arterial system is well developed, e.g.
On the other hand, in many Arthropods, especially those which possess tracheae, the arteries do not have a long course, but soon open into wide blood sinuses.
Watase, " On the Morphology of the Compound Eyes of Arthropods," Studies from the Biolog.
APTERA (Greek for "wingless"), a term in zoological classification applied by Linnaeus to various groups of wingless arthropods, including some of the insects, the centipedes, the millipedes, the Arachnida (scorpions, spiders, &c.) and the Crustacea.
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.
The general practice for many years past among naturalists has been to restrict the terms "Insecta" and "insect" to the class of Arthropods with three pairs of legs in the adult condition: bees, flies, moths, bugs, grasshoppers, springtails are "insects," but not spiders, centipedes nor crabs, far less earthworms, and still less slugs, starfishes or coral polyps.
A similar constitution of the body is more clearly seen in the Chaetopod worms. In the Vertebrata also a repetition of units of structure (myotomes, vertebrae, &c.) - which is essentially of the same nature as the repetition in Arthropods and Chaetopods, but in many respects subject to peculiar developments - is observed.
A satisfactory consideration of the structure of the Arthropods demands a knowledge of what may be called the laws of metamerism, and reference should be made to the article under that head.
From this ancestor Arthropods with heads of varying degrees of complexity have been developed characteristic of the different classes, whilst the parapodia and somites of the body have become variously modified and grouped in these different classes.
The structure of the head in Arthropods presents three profoundly separated grades of structure dependent upon the number of prosthomeres which have been assimilated by the prae-oral region.
The Chaetopod in Peripatus is not P, Protocerebrum or fore clearly ascertained, nor is its fate most cerebral mass be indicated by the study of the emlonging to the first bryonic head of other Arthropods so somite.
Which are like those of Chaetopods in (After Goodrich.) structure - viz.vesicles with an intravesicular lens, whereas the eyes of all other Arthropods have essentially another structure, being " cups " of the epidermis, in which a knob-like or rod-like thickening of the cuticle is fitted as refractive medium.
In connexion with the discussion of the limbs of Arthropods, a few words should be devoted to the gill-processes.
We are driven by the conclusions arrived at as to the derivation of the Arachnida from branchiate ancestors, independently of the other tracheate Arthropods, to formulate the conclusion that tracheae have been independently developed in the Arachnidan class.
This consideration leads us to one of the most remarkable and fascinating features of 'ant-communities - the presence in the nests of insects and other small arthropods, which are tended and cared for by the ants as their " guests," rendering to the ants in return the sweet food which they desire.
Exoskeleton The outer cellular layer (ectoderm or " hypodermis ") of insects as of other Arthropods, secretes a chitinous cuticle which has to be periodically shed and renewed during the growth of the animal.
- 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.
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.
We may conclude, therefore, that they were preceded, in Cambrian times or earlier, by Arthropods possessing well developed appendages on all the trunk-segments.
Of such Arthropods the living Symphyla - of which the delicate little Scutigerella is a fairly well-known example - give us some representation.
The agreement of the grouping of the somites, of the form of the parapodia (appendages, limbs) in each region, of the position of the genital aperture and operculum, of the position and character of the eyes, and of the powerful post-anal spines not seen in other Arthropods, is very convincing as to the affinity wise suppressed praegenital somite.
The first of these has, in Arachnids as in other Arthropods, its pair of appendages represented by the eyes.
The crural glands, which occur in many terrestrial Arthropods, are epidermal in origin and totally distinct from the coxal glands.
It has been considered by them as proving that Limulus, in spite of all its special agreements with Scorpio (which, however, have scarcely been appreciated by the writers in question), really belongs to the Crustacean line of descent, whilst Scorpio, by possessing Malpighian tubes, is declared to be unmistakably tied together with the other Arachnida to the tracheate Arthropods, the Hexapods, Diplopods, and Chilopods, which all possess Malpighian tubes.
On either side is attached a dorsolateral and ventro-lateral appendage, each with a fan-like plumose termination consisting of compound hairs or setae, found elsewhere only among arthropods (q.v.); each of these is moved by muscles running upwards towards the neck and arising immediately under the trochal disk, the inferior ventro-lateral pair also presenting muscles which form a girdle in the hind region of the body.
These experiments have shown that Arthropods also have their likes and dislikes in the matter of insect-food and frequently refuse to eat insects which are warningly coloured and are distasteful to vertebrated enemies.
It is amongst Arthropods, however - and especially amongst insects - that mimicry, both Batesian and Miillerian, occurs in greatest profusion and perfection.