Insects evolutionary development

Since insects are unable to biosynthesize the steroid nucleus, they require dietary sterols for structural and physiological (hormonal) purposes. Cholesterol will satisfy this dietary need in most cases, but since phytophagous insects ingest little or no cholesterol from dietary materials, they must convert dietary C28 and Q9 phytosterols to cholesterol or other sterols. Through evolutionary development, certain insects have acquired the ability to metabolize dietary sterols in unique ways and to produce and utilize a variety of ecdysteroids (molting hormones) for hormonal control of development and reproduction. Thus, insects are able to flourish in virtually every conceivable ecological niche. Certain comparative studies that illustrate these evolutionary processes will be discussed in this chapter. 

Insect mechanoreceptors and hair cells share evolutionary relationships. In flies, the proneural gene atonal controls development of mechanoreceptors Mathl plays a similar role in mice. Indeed, atonal can fully substitute for Mathl in mice and Mathl can fully substitute for atonal in flies [11], suggesting that these two disparate systems might share a common genetic program used for development. As another example, as we will discuss later, Drosophila bristle mechanoreceptors and zebrafish hair cells each rely on the TRP channel NompC for mechanotransduction. 

Resource restriction. If chemical defenses vary quantitatively within or between individual plants, then some tissues may be defended while others are not. As a result, insects have available to them the evolutionary option of avoidance they may develop the ability to recognize poor quality food and avoid it, rather than evolving detoxication mechanisms (12,18). This should result in feeding activity concentrated on a restricted set of tissues or plant individuals. There are two important consequences of this. First, contact rates with defenses can be lowered by avoiding them. Hence, the evolution of detoxication is less li)cely or less rapid (18). Second, and perhaps more important, the effectiveness of natural enemies may be enhanced (below). 

The Ametabola do not undergo metamorphosis. This is an evolutionary primitive condition and is exemplified by insects such as the bristletails and springtails. During development, these insects increase in size, but do not undergo distinct changes in form. In general, the Ametabola do not have wings. 

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