The history of the development of the head has been carefully worked out in the Hexapod insects.
Two divergent views have been held as to the nature of the original hexapod stock.
It is probably not a mere coincidence that the Hexapod, with its two rudimentary somites devoid of appendages, is thus found to possess twenty-one somites, including that which carries the anus, and that this is also the number present in the Malacostracous Crustacea.
Toothed so as to act as a biting jaw in the Hexapod Mantis, the Crustacean Squilla and others); with the last joint produced into a needle-like stabbing process in spiders.
The number is significant, since it agrees with that found in Edriophthalmous Crustacea, and assigns the labium of the Hexapod to the same somite numerically as that which carries the labium-like maxillipedes of those Crustacea.
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.
Lecaillon (1898) on various leaf beetles, tend to show that the organ " in the embryos of the lower Arthropoda corresponds with whole of the " mid-gut " arises from the proliferation of cells at the the region invaginated to form the serosa of the hexapod embryo.
Scorpio is here provided with a single or double pair of renal excretory tubes, which have been identified by earlier authors with the Malpighian tubes of the Hexapod and Myriapod insects.
That portion of the gut which is formed by the hypoblast, whereas in Hexapod insects the similar caecal tubes are developed from the proctodaeum or in -pushed portion of the gut which is formed from epiblast.
(From Lankester, " Limulus and Arachnid.") Galeodes has been made the means of a comparison between the structure of the Arachnida and Hexapod insects by Haeckel and other writers, and it was at one time suggested that there was a genetic affinity between the two groups - through Galeodes, or extinct forms similar to it.
The segmentation of the prosoma and the form of the appendages bear a homoplastic similarity to the head, pro-, meso-, and meta-thorax of a Hexapod with mandibles, maxillary palps and three pairs of walking legs; while the opistho io i e d c b o a S' S" 2 I VT V S IV III II I Opisthosoma Prosoma FIG.
(Original.) soma agrees in form and number of somites with the abdomen of a Hexapod, and the tracheal stigmata present certain agreements in the two cases.
But the word " Insect " had become limited since the days of Linnaeus to the Hexapod Pterygote forms, to the exclusion of his Aptera.
- Diagram of the head of a Hexapod insect.
An adult Hexapod is provided with a firm, well-chitinized cuticle and six conspicuous jointed legs.