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:
- age-related changes in physiology and morphology of the reproductive system;
- changes in somatic structures;
- 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 exopterygotes, 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.