External Structure

FIGURE 3.9. Simplified sectional diagram through the insect head showing the general arrangement of the parts. [From R. E. Snodgrass, Principles of Insect Morphology. Copyright 1935 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Company.]

Neuroptera and Coleoptera (with the exceptions mentioned below), Mecoptera, primitive Hymenoptera, and larval Ephemeroptera, Trichoptera, and Lepidoptera. However, the basic arrangement may undergo great modification associated with specialized feeding habits (especially the uptake of liquid food) or other, nontrophic functions. Suctorial mouthparts are found in members of the hemipteroid orders, and adult Siphonaptera, Diptera, higher Hymenoptera, and Lepidoptera. The mouthparts are reduced or absent in non-feeding or endoparasitic forms.

Examination of the structure of the mouthparts provides information on an insect's diet and feeding habits, and is also of assistance in taxonomic studies. Some of the more important modifications for the uptake of liquid food are described below. It will be noted that all sucking insects have two features in common. Some components of their mouthparts are modified into tubular structures, and a sucking pump is developed for drawing the food into the mouth.

Coleoptera and Neuroptera. In certain species of Coleoptera and Neuroptera the mouthparts of the larvae are modified for grasping, injecting, and sucking. In the beetle Dystiscus, for example, the laterally placed mandibles are long, curved structures with a groove having confluent edges on their inner surface (Figure 3.10). The labrum and labium are closely apposed so that the cibarium is cut off from the exterior. When prey is grasped, digestive fluids from the midgut are forced along the mandibular grooves and into the body. After external digestion, liquefied material is sucked back into the cibarium. In Dytiscus the suctorial pump is constructed from the cibarium, the pharynx, and their dilator muscles (see Figure 3.9).

Hymenoptera. In adult Hymenoptera a range of specialization of mouthparts can be seen. In primitive forms, such as sawflies, the mandible is a typical biting structure, and the maxillae and labium, though united, still exhibit their component parts. In the advanced forms, such as bees, the mandibles become flattened and are used for grasping and molding

FIGURE 3.10. Left mandible of Dytiscus larva, seen dorsally, showing the canal on its inner side. [From R. E. Snodgrass, Principles of Insect Morphology. Copyright 1935 by McGraw-Hill. Inc. Used with permission of McGraw-Hill Book Company.]

materials rather than biting and cutting. The maxillolabial complex is elongate and the glossae form a long flexible "tongue," a sucking tube capable of retraction and protraction (Figure 3.11). The laciniae are lost and the maxillary palps reduced, but the galeae are much enlarged, flattened structures, which in short-tongued bees are used to cut holes in the flower corolla to gain access to the nectary. When the food is easily accessible, the glossae, labial palps, and the galeae form a composite tube up which the liquid is drawn. When the food is confined in a narrow cavity such as a nectary, only the glossae are used to obtain it. The sucking mechanism of the Hymenoptera includes the pharynx, buccal cavity, and cibarium, and their dilator muscles.

Lepidoptera. Functional biting mouthparts are retained in the adults of only one family of Lepidoptera, the Micropterigidae. In all other groups the mouthparts (Figure 3.12) are considerably modified in conjunction with the diet of nectar. The mandibles are usually lost, the labrum is reduced to a narrow transverse sclerite, and the labium is a small flap

Insects Mouthpart Structure

FIGURE 3.11. Mouthparts of the honey bee. [After R. E. Snodgrass. 1925, Anatomy and Physiology of the Honey bee, McGraw-Hill Book Company.]

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FIGURE 3.12. Head and mouthparts of Lepidoptera. (A) General view of the head and (B) cross-section of the proboscis. [From R. E. Snodgrass, Principles of Insect Morphology. Copyright 1935 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Company.]

(though its palps remain quite large). The long, suctorial proboscis is formed from the interlocking galeae, whose outer walls comprise a succession of narrow sclerotized arcs alternating with thin membranous areas: presumably this arrangement facilitates coiling. Extension of the proboscis is brought about by a local increase in blood pressure. The sucking pump of Lepidoptera comprises the same elements as that of Hymenoptera. In Lepidoptera that do not feed as adults all mouthparts are greatly reduced and the pump is absent.

Diptera. In both larval and adult Diptera the form and function of the mouthparts have diverged considerably from the typical chewing condition. Indeed, in extreme cases [seen in some of the larvae (maggots) of Muscomorpha] it appears that not only a new feeding mechanism but an entirely new functional head and mouth have evolved, the true mouthparts of the adult fly being suppressed during the larval period. This remarkable modification of the head and its appendages is, of course, the result of the insect living entirely within its food.

Larva. In larvae of many orthorrhaphous flies the head is retracted into the thorax and enclosed within a sheath formed from the neck membrane. The mandibles and maxillae possess the typical biting structure (though the palps are small or absent). The labrum is large and overhanging. The labium is rudimentary and often confused with the hypostoma, a toothed, triangular sclerite on the neck membrane (Figure 3.13A-C).

In maggots the true head is completely invaginated into the thorax, and the conical "head" is, in fact, a sclerotized fold of the neck. The functional "mouth" is the inner end of the preoral cavity, the atrium, from which a pair of sclerotized hooks protrude. The cibarium is transformed into a massive sucking pump, and the true mouth is the posterior exit from the pump lumen (Figure 3.13D).

Adult. No adult Diptera have typical biting mouthparts, although, of course, many blood feeders are said to "bite" when they pierce the skin. The mouthparts can be divided functionally into those that only suck and those that first pierce and then suck. In the latter the piercing structure may be the mandibles, the labium, or the hypopharynx.

In Diptera that merely suck or "sponge" up their food (e.g., the house fly and blow fly) the mandibles have disappeared and the elongate feeding tube, the proboscis, is a composite structure that includes the labrum, hypopharynx, and labium (Figure 3.14). The proboscis

FIGURE 3.13. Head and mouthparts of larval Diptera. (A) Diagrammatic section through the retracted head of Tipula; (B) right mandible of Tipula; (C) left maxilla of Tipula; and (D) diagrammatic section through the anterior end of a maggot. [From R. E. Snodgrass, Principles of Insect Morphology. Copyright 1935 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Company.]

FIGURE 3.13. Head and mouthparts of larval Diptera. (A) Diagrammatic section through the retracted head of Tipula; (B) right mandible of Tipula; (C) left maxilla of Tipula; and (D) diagrammatic section through the anterior end of a maggot. [From R. E. Snodgrass, Principles of Insect Morphology. Copyright 1935 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Company.]

is divisible into a basal rostrum bearing the maxillary palps, a median flexible haustellum, and two apical labella. The latter are broad sponging pads, equipped with pseudotracheae along which food passes to the oral aperture. The latter is not the true mouth, which lies at the upper end of the food canal. As in other Diptera, the sucking apparatus is formed from the cibarium and its dilator muscles that are inserted on the clypeus.

Many Diptera feed on blood. Some of these (e.g., the tsetse fly, stable fly, and horn fly), like their non-piercing relatives, have a composite proboscis. However, the haustellum is elongate and rigid, and the distal labellar lobes are small but bear rows of prestomal teeth on their inner walls. The labrum and labium interlock to form the food canal within which lies the hypopharynx enclosing the salivary duct (Figure 3.15).

Other blood-feeding flies (e.g., horse flies, deer flies, black flies, and mosquitoes) use the mandibles for piercing the host's skin. The mouthparts of the horse fly Tabanus may be

Chewing Mouth Parts Insect

FIGURE 3.14. Head and mouthparts of the house fly. (A) Lateral view of the head with the proboscis extended; and (B) anterodistal view of the proboscis. [From R. E. Snodgrass, Principles of Insect Morphology. Copyright 1935 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Company.]

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Beekeeping for Beginners

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