No new theory is introduced here, but critical attention will be paid to some current theories about the segmentation of the insect head. Elongated animals that usually move in one direction necessarily have their main sense organs at the front of the body. Thus the front part of the animal structurally became a head bearing the orienting sense organs and the mouth.
From all this it is evident that a number of advanced ideas have been involved in the organization of the insect's head which it does. The embryonic mouth (Mth) forms in the abdomen at the base of the inner growth of the cephalic lobe of the anectodermal stomodeum. In front of the mouth the labrum (Lm) usually appears as a small lobe on the underside of the head.
It contains the centers of the optic and antennal nerves, which become the protocerebrum and deutocerebrum of the definitive brain. The main proof of the existence of this segment is the presence of the premandibular pair.
NO. I THE INSECT HEAD — SNODGRASS 5 ganglia that become the tritocerebral lobes of the definitive insect
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GENERAL EXTERNAL STRUCTURE OF THE INSECT HEAD
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The groove perhaps most common on the head is one that crosses the lower part of the face and forms a strong internal brace between the anterior articulation of the mandibles. In others it is incomplete, and it may even be absent in insects with strong, jaw-like mandibles, in which case the frontal and clypeal regions are continuous (fig. 4A). The head area known as the frons (fig. 6D, Fr) can only be defined as the facial region between the compound eyes and the antennae, extending to the clypeus.
These lines, however, as will be shown later (Fig. 7), are so variable in the position they occupy that they do not define any particular part of the face. Dorsally, the frontal part passes without interruption into the curved top of the head, known as the vertex (D, E, Vx). The vertex and dorsal part of the forehead are sometimes marked by a median cranial sulcus (A, mcs) and a pair of lateral temporal sulci (ts) converging between the compound eyes.
Beneath each eye complex is softened the subocular sulcus (fig. 6A, B,sos), which when present separates the ends from the lateral wall of the head called the gena (D, E, Ge). Since the mandibles and maxillae are articulated at the lower ends of the genae,
NO. I THE INSECT HEAD — SNODGRASS 17 these margins are commonly reinforced by submarginal internal ridges
At the back of the head the postoccipital sulcus (fig. 6 B, pos), as already described, leaves the narrow postoccipital pit (E, F, Poc) which arches over the occipital foramen (F, For). The area behind this sulcus is known as the occiput whether the sulcus is present or not. Close around the compound eye is usually a circumocular sulcus (fig. 6 A, B, cos) which reinforces the cranial edge of the eye.
Finally, in different insects, different linear grooves may appear on the head that are not related to each other, because they are independent adaptations to a special need of the specific species. Yet it has long been described as an important structural feature of the insect head. Y seen from the front (fig. 7A, B, CL), with the stem on top of the head and the arms spread downwards.
In ecdysis, the entire line breaks up (C, D, E) and the stalk is then seen to be continuous from the median cleavage line on the back of the thorax (E). The facial area between the arms of the cleavage line is more softly identified as the frons, but these lines do not consistently define any anatomical part of the head, as they vary widely in their extent and position in different insects. Typically they extend down to the clypeus, but in some insects they turn laterally, and in ecdysis the slits cut through the compound eyes (Fig. 7 E), in others the arms.
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THE POSTERIOR HEAD STRUCTURE
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THE ANTENNAE
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CHAPTER I INSECTS — SNODGRASS 31called flagellomeres, as suggested by Imius (1940), but they are called flagellomeres, as suggested by Imius (1940), but they are not "antennal segments". In adult Pterygota birds, the antennae assume many different forms, which are mainly produced by modifications of the flagellum or by the differentiation of its annuHs. In lamellicorn beetles, some of the distal rings are produced perpendicular to the stem as overlapping leaf-like plates (C).
In the muscoid antenna (F, G) it . the flagellomere becomes a large oval lobe {ifi) resting on the stalk, and from this a flagellumireduced toanarist (Ar), consisting of two small basal rings and a long, tapering, simple or usually branched distal stalk. The antennae of holometabolic larvae are often so different from those of adults that they look like special larval organs rather than developmental stages of adult antennae. Antennae of four or five units occur in Megaloptera (B) and in Cantharidae, Dytiscidae, and Hydrophilidae among Coleoptera, but the antennae of most larval beetles are very small, three-segmented organs (F).
Similarly, the antennae of lepidopterous larvae (D) have only three segments, the third being a mere apical lobe on the second (E). In Chironomus the antenna (C) may have three short apical units on a long base, but in the mosquito larva the very small antenna (G) is undivided. In the higher Hymenoptera the larval antennae are represented by only slight swellings or mere discs of the head wall.
In the muscoid fly larva, the antennae are completely eliminated externally, as they are formed in a pair of long sacs from the frontal region of the head that extend back into the thorax. When the larval antenna is greatly reduced in size, the next pupal antenna develops either under the cuticle of the head or under the cuticle of the head. For example, in the mosquito, as shown by Imms (1908), the pupal antenna is formed in a deep pocket on the head, but has no connection with the larval organ.
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THE TENTORIUM
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THEORETICAL CONSIDERATIONS
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THE LABRUM
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I HEAD OF INSECTS — SNODGRASS 43nerves arise from the sensory cells of the epidermis and enter the tritonerve nerves originate from the sensory cells of the epidermis and enter the tritocerebral ganglia usually via the frontal ganglionic ligaments. Dorsal tegumentary nerves from the insect tritocerebrum undoubtedly go to non-tritocerebral segmental regions of the head, as this segment is practically eliminated in the adult head. In the malacostracan Crustacea, according to Hanstrom (1928), the dorsal tegumentary nerve branches from the brain anteriorly to the eye stalks and posteriorly to the entire cephalothorax.
In Limulus, Patten and Redenbaugh (1899) describe and illustrate a pair of lateral nerves from the tritocerebral ganglia that turn back on the epidermis of the leg segments and finally branch toward the first five abdominal appendages. The neurocytes of the motor nerves, however, lie in the ganglia and, in general, their function is to innervate the muscles of the corresponding body segments. Niiesch (1954) reports that in the thorax of the moth Telea polyphenms the second ganglion gives off nerves to three thoracic segments.
Therefore, the deductions of Henry and Buttare are not justified, since both are based on the sensory nerves of the labrum, which are simply a bundle of sensory fibers from the general preoral region of the head. Only in the tritocerebral ganglion can these fibers make connections with motor neurons of the ventral nerve cord. Of greater importance than the sensory innervation of the anterior parts of the head is the fact that the motor innervation of the labral.
Chaudon-neret (1950), for example, described in Thermohia domestica an elaborate innervation of the anterior region of the head by the frontal ganglion.
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THE EMBRYONIC HEAD LOBE
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THE ANTENNAE
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NO. I THE INSECT HEAD — SNODGRASS 55 In further experiments Herbst found that in the Crustacea the
LETTERING ON THE FIGURES
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