Agegrading

Identification of the growth stages or ages of insects in a population is important in ecological or applied entomology. Information on the proportion of a population in different developmental stages and the proportion of the adult population at reproductive maturity can be used to construct time-specific life-tables or budgets to determine factors that cause and regulate fluctuations in population size and dispersal rate, and to monitor fecundity and mortality factors in the population. Such data are integral to predictions of pest outbreaks as a result of climate and to the construction of models of population response to the introduction of a control program.

Many different techniques have been proposed for estimating either the growth stage or the age of insects. Some provide an estimate of chronological (calendar) age within a stadium, whereas most estimate either instar number or relative age within a stadium, in which case the term age-grading is used in place of age determination.

6.9.1 Age-grading of immature insects

For many population studies it is important to know the number of larval or nymphal instars in a species and to be able to recognize the instar to which any immature individual belongs. Generally, such information is available or its acquisition is feasible for species with a constant and relatively small number of immature instars, especially those with a lifespan of a few months or less. However, it is logistically difficult to obtain such data for species with either many or a variable number of instars, or with overlapping generations. The latter situation may occur in species with many asynchronous generations per year or in species with a life cycle of longer than one year. In some species there are readily discernible qualitative (e.g. color) or meristic (e.g. antennal segment number) differences between consecutive immature instars. More frequently, the only obvious difference between successive larval or nymphal instars is the increase in size that occurs after each molt (the molt increment). Thus, it should be possible to determine the actual number of instars in the life history of a species from a frequency histogram of measurements of a sclerotized body part (Fig. 6.11).

Entomologists have sought to quantify this size progression for a range of insects. One of the earliest

Fig. 6.11 Growth and development in a marine midge, Telmatogeton (Diptera: Chironomidae), showing increases in: (a) head capsule length; (b) mandible length; and (c) body length between the four larval instars (I-IV). The dots and horizontal lines above each histogram represent the means and standard deviations of measurements for each instar. Note that the lengths of the sclerotized head and mandible fall into discrete size classes representing each instar, whereas body length is an unreliable indicator of instar number, especially for separating the third- and fourth-instar larvae.

Fig. 6.11 Growth and development in a marine midge, Telmatogeton (Diptera: Chironomidae), showing increases in: (a) head capsule length; (b) mandible length; and (c) body length between the four larval instars (I-IV). The dots and horizontal lines above each histogram represent the means and standard deviations of measurements for each instar. Note that the lengths of the sclerotized head and mandible fall into discrete size classes representing each instar, whereas body length is an unreliable indicator of instar number, especially for separating the third- and fourth-instar larvae.

attempts was that of H.G. Dyar, who in 1890 established a "rule" from observations on the caterpillars of 28 species of Lepidoptera. Dyar's measurements showed that the width of the head capsule increased in a regular linear progression in successive instars by a ratio (range 1.3-1.7) that was constant for a given species. Dyar's rule states that:

postmolt size/premolt size (or molt increment)

= constant

Thus, if logarithms of measurements of some sclerot-ized body part in different instars are plotted against the instar number, a straight line should result; any deviation from a straight line indicates a missing instar. In practice, however, there are many departures from Dyar's rule, as the progression factor is not always con stant, especially in field populations subject to variable conditions of food and temperature during growth.

A related empirical "law" of growth is Przibram's rule, which states that an insect's weight is doubled during each instar and at each molt all linear dimensions are increased by a ratio of 1.26. The growth of most insects shows no general agreement with this rule, which assumes that the dimensions of a part of the insect body should increase at each molt by the same ratio as the body as a whole. In reality, growth in most insects is allometric, i.e. the parts grow at rates peculiar to themselves, and often very different from the growth rate of the body as a whole. The horned adornments on the head and thorax of Onthophagus dung beetles discussed in section 5.3 exemplify the trade-offs associated with allometric growth.

6.9.2 Age-grading of adult insects

The age of an adult insect is not determined easily. However, adult age is of great significance, particularly in the insect vectors of disease. For instance, it is crucial to epidemiology that the age (longevity) of an adult female mosquito be known, as this relates to the number of blood meals taken and therefore the number of opportunities for pathogen transmission. Most techniques for assessing the age of adult insects estimate relative (not chronological) age and hence age-grading is the appropriate term.

Three general categories of age assessment have been proposed, relating to:

1 age-related changes in physiology and morphology of the reproductive system;

2 changes in somatic structures;

3 external wear and tear.

The latter approach has proved unreliable but the other methods have wide applicability.

In the first method, age is graded according to reproductive physiology in a technique applicable only to females. Examination of an ovary of a parous insect (one that has laid at least one egg) shows that evidence remains after each egg is laid (or even resorbed) in the form of a follicular relic that denotes an irreversible change in the epithelium. The deposition of each egg, together with contraction of the previously distended membrane, leaves one follicular relic per egg. The actual shape and form of the follicular relic varies between species, but one or more residual dilations of the lumen, with or without pigment or granules, is common in the Diptera. Females that have no follicular relic have not developed an egg and are termed nulliparous.

Counting follicular relics can give a comparative measure of the physiological age of a female insect, for example allowing discrimination of parous from nulliparous individuals, and often allowing further segregation within parous individuals according to the number of ovipositions. The chronological age can be calculated if the time between successive ovipositions (the ovarian cycle) is known. However, if there is one ovarian cycle per blood meal, as in many medically significant flies, it is the physiological age (number of cycles) that is of greater significance than the precise chronological age.

The second generally applicable method of age determination has a more direct relationship with chronology, and most of the somatic features that allow age estimation are present in both sexes. Estimates of age can be made from measures of cuticle growth, fluorescent pigments, fat body size, cuticular hardness and, in females only, color and/or patterning of the abdomen. Cuticular growth estimates of age rely upon there being a daily rhythm of deposition of the endocuticle. In exo-pterygotes, cuticular layers are more reliable, whereas in endopterygotes, the apodemes (internal skeletal projections upon which muscles attach) are more dependable. The daily layers are most distinctive when the temperature for cuticle formation is not attained for part of each day. This use of growth rings is confounded by development temperatures too cold for deposition, or too high for the daily cycle of deposition and cessation. A further drawback to the technique is that deposition ceases after a certain age is attained, perhaps only 10-15 days after eclosion. Physiological age can be determined by measuring the pigments that accumulate in the aging cells of many animals, including insects. These pigments fluoresce and can be studied by fluorescence microscopy. Lipofuscin from postmitotic cells in most body tissues, and pteridine eye pigments have been measured in this way, especially in flies.

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