The number of antennal segments varies from eleven to two.
Antennal segment s form a distinct club.
This is a very well-marked tribe of beetles, characterized by the peculiar elongation and flattening of three or more of the terminal antennal segments, so that the feeler seems to end in a number of leaf-like plates, or small comb-teeth (fig.
I and 25) have the terminal antennal segments pectinate, and so arranged that the comb-like part of the feeler cannot be curled up, while the elytra completely cover the abdomen.
In this family there is often a marked divergence between the sexes; the terminal antennal segments are larger in the male than in the female, and the males may carry large spinous processes on the head or prothorax, or both.
The antennae of Diptera, which are also extremely important in classification, are thread-like in the more primitive families, such as the Tipulidae (daddy-long-legs), where they consist of a considerable number of joints, all of which except the first two, and sometimes also the last two, are similar in shape; in the more specialized families, such as the Tabanidae (horse-flies), Syrphidae (hover-flies) or Muscidae (house-flies, blue-bottles and their allies), the number of antennal joints is greatly reduced by coalescence, so that the antennae appear to consist of only three joints.
Behind the antennal (or deutocerebral) segment an "intercalary " or tritocerebral segment has been demonstrated by W.
All of these are to be regarded as primitively post-oral, but in the course of development the mouth moves back to the mandibular segment, so that the first three somitesocular, antennal and intercalary - lie in front of it.
17), is the mouth or oral piece; the second, explained by the presence of a " latent endoderm-group " in those the antennal segment; the third, the intercalary or prae-mandibular invaginations.
The antennal segment apparently entirely disappears, with the exception of a pair of appendages it bears; these become the antennae; it is possible that the original segment, or some part of it, may even become a portion of the actual antennae.
One of these is the ocular segment, in front of the antennal, and behind the primitive preoral segment.
- A, Front of head of Sawfly (Pachynematus); a, labrum; b, clypeus; c, vertex; d, d, antennal cavities.
X Ocelli and post-antennal organ of right side.
TipX terminal antennal segment with antennal organ.
In many genera of springtails a curious post-antennal organ, consisting of sensory structures (often complex in form) surrounded by a firm ring, is to be noticed on the cuticle of the head between the eyes and the feelers.
Other sensory organs occur on the third and fourth antennal segments in the Achorutidae and Entomobryidae (fig.
And xxiv., 1900, 1901), the two latter writers having paid especial attention to the peculiar postantennal and antennal sense-organs of springtails.
Thus, in the Phyllopoda, the antennal gland develops early and is functional during a great part of the larval life, but it ultimately atrophies, and in the adult (as in most Entomostraca) the maxillary gland is the functional excretory organ.
In the Decapoda, where the antennal gland alone is well-developed in the adult, the maxillary gland sometimes precedes it in the larva.
In the Decapoda the antennal gland is largely developed and is known as the " green gland."
The centres for the antennal nerves form ganglionic swellings on the oesophageal connectives.
In the higher forms, as already mentioned, the antennal ganglia have become shifted forwards and coalesced with the brain.