Reproduction

FIGURE 19.6. Egg of Locusta. (A) General view; (B) enlargement of posterior end; (C) section through chorion along micropylar axis; and (D) details of chorion structure. [A, after R. F. Chapman, 1971, The Insects: Structure and Function. By permission of Elsevier/North-Holland, Inc., and the author. B, C, after M. L. Roonwal, 1954, The egg-wall of the African migratory locust, Locusta m. migratorioides R&F. (Orthoptera: Acrididae), Proc. Natl. Inst. Sci. India 20:361-370. By permission of the Indian National Science Academy. D, after J. C. Hartley, 1961, The shell of acridid eggs, Q. J. Microsc. Sci. 102:249-255. By permission of Cambridge University Press.]

FIGURE 19.6. Egg of Locusta. (A) General view; (B) enlargement of posterior end; (C) section through chorion along micropylar axis; and (D) details of chorion structure. [A, after R. F. Chapman, 1971, The Insects: Structure and Function. By permission of Elsevier/North-Holland, Inc., and the author. B, C, after M. L. Roonwal, 1954, The egg-wall of the African migratory locust, Locusta m. migratorioides R&F. (Orthoptera: Acrididae), Proc. Natl. Inst. Sci. India 20:361-370. By permission of the Indian National Science Academy. D, after J. C. Hartley, 1961, The shell of acridid eggs, Q. J. Microsc. Sci. 102:249-255. By permission of Cambridge University Press.]

region which ruptures to facilitate hatching, while in the eggs of parasitic Hymenoptera the membrane is extremely thin to permit uptake of nutrients from the host's hemolymph. In D. melanogaster the follicle cells secrete wax immediately after production of the vitelline membrane. The wax layer is found in eggs of many species and, like that of the cuticle, prevents desiccation. A wax layer is not found, however, in eggs that are normally laid in humid or wet microclimates or that take up water from the environment prior to embryonic development.

The chorion is usually secreted entirely by the follicle cells and comprises two main layers, an endochorion adjacent to the vitelline membrane and an exochorion (Figure 19.6). In some insects, for example, Acrididae, the shell takes on a third layer, the extrachorion,

FIGURE 19.7. Diagrammatic sagittal section through an egg at oviposition. [After R. F. Chapman, 1971, The Insects: Structure and Function. By permission of Elsevier/North-Holland, Inc., and the author.]

as an oocyte moves through the common oviduct. Interestingly, though the follicle cells are mesodermal derivatives, the chorion is cuticlelike in nature and contains layers of protein and lipoprotein (but not chitin), some of which are tanned by polyphenolic substances released by the cells.

The chorion is not produced as a uniform layer over the oocyte. For example, in some species a ring of follicle cells near the anterior end of the oocyte secrete no exochorion, so that a line of weakness is created at this point for ease of hatching. Also, certain follicle cells appear to have larger than normal microvilli which, when withdrawn after chorion formation, leave channels (micropyles) to permit entry of sperm (Figure 19.6C,D). The aeropyles (air canals) appear to be formed in a similar way. In eggs of many species, both terrestrial and aquatic, the aeropyles connect with a network of minute air spaces within the endochorion (the plastron), which improves gas exchange between the oocyte and the atmosphere. The plastron in eggs of terrestrial species usually serves to prevent desiccation caused by evaporation; for aquatic insects (or terrestrial species whose eggs become immersed temporarily in water, for example, after rain), the plastron prevents waterlogging of the egg while facilitating gas exchange (see Chapter 15, Section 4.2). Like the vitelline membrane, the chorion varies in thickness among species. In the cecropia moth, Hyalophora cecropia, it is 55-60 |am, providing a rigid protective coat, while in parasitic Hymenoptera it is very thin (e.g., Habrobracon juglandis, 0.17 |^m) and flexible to permit stretching of the egg during its passage along the very narrow ovipositor.

The internal structure of a mature (i.e., chorionated) egg is shown diagrammatically in Figure 19.7.

3.1.3. Factors Affecting Sexual Maturity in the Female

Environmental. A number of environmental factors have been observed to influence the rate at which a female becomes sexually mature, for example, quantity and quality of food consumed, population density and structure, mating, photoperiod, temperature, and humidity. For most factors there is evidence that they exert their influence by modifying the activity of the endocrine system, though availability of food and temperature also have obvious direct effects on the rate of egg development.

Many reports indicate that the qualitative nature of food may have a marked effect on the number of eggs matured. However, it is often not clear whether the observed differences in egg production are related to differences in palatability (more palatable foods might be 573

eaten in larger quantities) or to differences in the nutritive value of the food. For most

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