Chapter

488 to oviposition. In the migratory locust, on which much work has been done, olfaction is of primary importance in food location. Once the insect makes contact with the vegetation, tarsal chemosensilla initiate a reflex that results in the stoppage of movement. Sensilla on the labial and maxillary palps then taste the surface waxes of the plant, after which the locust takes a small bite. Whether feeding continues is sometimes determined by mechanosensillar responses to physical stimuli such as the hardness, toughness, shape, and hairiness of the food. More commonly, it is substances in the released sap that, by stimulating chemosensilla in the cibarial cavity, regulate the continuation or arrest of feeding (Chapman, 2003). These substances are called "phagostimulants" or "deterrents," respectively. The substances may have nutritional value to the insect or may be nutritionally unimportant ("token stimuli"). Nutritional factors are almost always stimulating in effect. Sugars, especially sucrose, are important phagostimulants for most phytophagous insects. Amino acids, in contrast, are generally by themselves weakly stimulating or non-stimulating, though may act synergistically with certain sugars or token stimuli. For example, Heron (1965) showed in the spruce bud-worm (Choristoneurafumiferana) that, whereas sucrose and L-proline in low concentration were individually only weak phagostimulants, a mixture of the two substances was highly stimulating. In addition to sugars and amino acids, other specific nutrients may stimulate feeding in a given species. Such nutrients include vitamins, phospholipids, and steroids. Token stimuli may either stimulate or inhibit feeding. Thus, derivatives of mustard oil, produced by cruciferous plants, including cabbage and its relatives, are important phagostimulants for a variety of insects that normally feed on these plants, for example, larvae of the diamondback moth (Plutella xylostella), the cabbage aphid (Brevicoryne brassicae), and the mustard beetle (Phaedon cochleariae). Indeed, Plutella will feed naturally only on plants that contain mustard oil compounds. Many secondary plant metabolites, including alkaloids, terpenoids, phenolics, and glycosides, are feeding deterrents for phytophagous insects. In a given food source there will probably be a mixture of phagostimulants and deterrents, and the balance of this sensory input, integrated through the central nervous system, determines the overall palatability of the food.

Species whose choice of food is limited are said to be oligophagous. In extreme cases, an insect may be restricted to feeding on a single plant species and is described as monophagous. Species that may feed on a wide variety of plants are polyphagous, though it must be noted that even these exhibit selectivity when given a choice. Not surprisingly, monophagous and oligophagous species are especially sensitive to the presence of deterrents in non-host plants.

In many predaceous insects, especially those that actively pursue prey, vision is of primary importance in locating and capturing food. As noted in Chapter 12 (Section 7.1.2), some predaceous insects have binocular vision that enables them to determine when prey is within catching distance. Carnivorous species, especially larval forms, whose visual sense is less well developed, depend on chemical or tactile stimuli to find prey. For example, many beetle larvae that live on or in the ground locate prey by their scent. Species parasitic on other animals usually locate a host by its scent, though tsetse flies may initially orient by visual means to a potential host. For many species that feed on the blood of birds and mammals, temperature and/or humidity gradients are important in determining the precise location at which an insect alights on a host and begins to feed.

The extent of food specificity for carnivorous insects is varied. Many insects are quite non-specific and will attempt to capture and eat any organism that falls within a given size range (even to the extent of being cannibalistic). Others are more selective; for example, spider wasps (Pompilidae), as their name indicates, capture only spiders for provisioning

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

Get My Free Ebook


Post a comment