Invertebrate Estivation A seemingly lifeless desert may be teeming with estivating life underground, waiting for seasonal rains that will awaken them to resume their life cycles. Snails, slugs, earthworms, insects, spiders, and nematodes, along with cocoons, eggs, grubs, larvae, and pupae, may all lie dormant in the soil, in building foundations and rock crevices, or under rotting logs or other vegetation. The animals are not just dormant; some of them will also be in an arrested state of sexual development called diapause. Some tropical snails can estivate for years at a time. To prepare, a snail digs a deep burrowin moist ground or under rocks. Next, it forms an epiphragm (a sealing membrane for the shell) to prevent evaporation and desiccation. Finally, its metabolism slows dramatically until it detects favorable environmental cues such as increased moisture.
Color Vision Reflected natural light has a unique property that maybe exploited by the visual system of an animal, namely its wavelength, or color. Color vision can be vital for a species in regard to mate choice and foraging. Photoreceptors can be sensitive to different wavelengths of light, or colors; in vertebrates the color-sensitive receptors are called cones because of their shape. The maximum sensitivity of a photoreceptor is dictated by the nature of the photo-sensitive protein (opsin) within the receptor. Opsins can be classified according to the approximate wavelength of light that stimulates them maximally. Opsins have been studied that are sensitive to light fromthe ultraviolet region of the visible spectrum all the way to the far red region. To have the possibility of color vision, an animal must possess at least two photoreceptors with differing sensitivities. The brain must then be able to compare the outputs of both these receptors and discriminate color. Often more than two types of photopigment type are present, as in the fish retina, which results in very complex color vision, including sensitivity to ultraviolet light. Color vision has been shown to exist in many animals within the animal kingdom. Positioning and distribution of the different types of photoreceptor within the retina are also key to the ability to discriminate color. In vertebrates, this aspect of retinal structure is called the cone mosaic, and its nature is often closely related to some behavioral aspect of the animal in question. Color vision also requires a central processing system that can decode the various light signals and turn them into a brain output which is useful to the animal.
