Measuring and Reporting Life-Cycle Duration in Insects and Arachnids
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REVIEW Eur. J.Entomol. 97: 285-303, 2000 ISSN 1210-5759 Measuring and reporting life-cycle duration in insects and arachnids Hugh V. DANKS Biological Survey of Canada (Terrestrial Arthropods), Canadian Museum ofNature, P.O. Box 3443, Station “D”, Ottawa, Ontario, Canada K1P 6P4; e-mail: [email protected] Key words.Life-cycles, duration, rearing, mstar, stage, demography, experimental design, statistics, insects, mites Abstract. Some previous work on arthropod development is insufficiently detailed or incompletely reported. Much of the published information in this area is of limited use for the general analysis of life cycles. These difficulties arise primarily because many experiments do not control fully for the strain of the material (and even its specific identity) nor for rearing conditions, do not ade quately take account of the complexity of life cycles and their stages, or are restricted to only part of the life cycle. For example, 285 such factors as variable numbers of instars, sexual differences, abbreviated or hidden stages and dormancies may mean that the “average durations” reported apply to an unknown mixture of developmental types. Nor are experiments always designed or results reported and analysed in a logical and transparent manner. Undefined terms may obscure what actual developmental intervals were measured. Highly derived developmental or demographic measures may obscure core data. Statistical information may be inade quate. Such pitfalls are reviewed here, suggesting ways to ensure that results on the duration of development are both valid for spe cific studies and more widely useful. General experimental difficulties, recommended background information that should be provided, recommended life-cycle intervals and their terminology, and recommended ways to report numerical and statistical infor mation are briefly summarized in tabular form. INTRODUCTION STAGES OF THE LIFE CYCLE How long does an insect’s life cycle and its component Characteristic stages of the insect life-cycle are shown stages last? The answer to that question, including how in Fig. 1, emphasizing immature stages. Exopterygotes these durations are modified when conditions of tempera omit the prepupal and pupal stages, of course. These basic ture, food or other factors change, is a key piece of infor stages are patterned in many different ways in different mation for understanding the way of life of every species. groups, including the omission or abbreviation of par But the question is surprisingly complex, and this paper ticular substages. The instars may be very similar to or examines the difficulties in defining stages, and in very different from one another, and may include one or securing and expressing data on life-cycle duration. It more modified resting stages. suggests some procedures to be adopted or avoided for all In typical mites (as in tetranychids, for example), the kinds of studies, based on my overview of how key successive immature stages are the egg, larva (usually six aspects of insect life cycles can best be summarized legged), protonymph and deutonymph. However, as in (Danks, 1987, 1991, 1992, 1994,unpubl. analysis). insects, different groups of mites have different life-cycle Some of the conventions adopted to express durations patterns. For example, some species have an additional of development during the life cycle differ among indi nymphal stage; in others one or two stages are lost. viduals, groups of scientists, and even journals. In addi Egg stage.After oviposition, the egg hardens and may tion, much of the information published is inadequate or change colour. Other colour changes, especially darken incomplete, for various reasons. I have cited relatively ing, are visible before hatch. The duration of embryo few specific examples of such inadequacies here, because genesis is correlated with taxonomic placement (Order) it is not my intention to embarrass particular authors. (Detlaff, 1996). Many stages of embryonic development Moreover, procedures and summaries depend on the pur can be recognized by careful examination (Miya, 1965 for pose of a particular experiment, which may have different chrysomelids; Ingrisch, 1984 for tettigoniids; Gotoh et al., needs than for the general analysis of life cycles. Never 1994 for mites), and in a more general way from con theless, I argue here that data should be presented in ways spicuous features visible externally, such as coloration or that enhance their broader value. eyespot development. If development is delayed it can This paper aims to present a variety of relevant terms, often be detected from such markers. In several species a difficulties and recommendations in one place. I hope resting stage immediately precedes hatch, so that the egg thereby to encourage entomologists conducting the hun contains a fully formed pharate larva (e.g. Lymantria dreds of studies carried out each year on the duration of dispar L.). Hatch itself (exit of the larva from the insect development to ensure that their results are both chorion) usually is relatively rapid, although in some spe valid and widely useful. By this means, they can be built cies the first instars remain for some time within struc upon to advance knowledge (like all properly published tures associated with the eggs, such as a gelatinous matrix research) and not simply be “one-time” products of lim ited value. 285 Fig. 1. Scheme showing visible stages of development in a typical endopterygote insect, emphasizing immature stages. (e.g. Danks, 1971 for chironomids; Wiggins, 1973 for flies (Vaught & Stewart, 1974), dragonflies (Rivard & limnephilid caddisflies). Pilon, 1977), grasshoppers (Kawano & Ando, 1997), bee Larval stage.The major role of larvae in the life cycle tles (Beck, 1971; Gerard & Ruf, 1997), moths (Neunzig, is to feed and grow. Consequently, their structural and 1969; Morita & Tojo, 1985; Clare & Singh, 1990; temporal patterns of development are related largely to Kamata & Igarashi, 1995), and other groups. The devel the need to survive in particular environments (generally opmental routes of individuals with different numbers of chosen by ovipositing females) and to acquire and use the instars are not equivalent, and data on instar durations food resources there. have to be recorded separately for these different indi In many species first instars differ from later instars, viduals. especially if they serve for dispersal (as in the planktonic At any given time, each instar can be in one of three first instars of Chironomidae: Davies, 1976), host finding states: active and feeding; inactive and moulting; or inac (as in parasitoids that penetrate hosts from the host tive and dormant. Moults can occupy a significant frac habitat) or protection of the resource from other indi tion of the time spent in the larval period, especially at viduals (as in the hyper-metamorphic first instars of many lower temperatures; e.g. 43% of the penultimate instar in parasitic hymenopterans). Typically, first instars moult the geometrid moth Epirrita autumnata (Borkhausen) and relatively rapidly to the second instar, and in some species 50% in the chrysomelid beetle Galerucella sagittariae do not feed [Harvey, 1957 for the tortricid moth Choristo- Gyllenhal at 12°C (Ayres & Maclean, 1987). Moreover, neura fumiferana (Clem.); Strickman et al., 1997 for the larval moults, pupation and adult eclosion are prevented pholcid spider Crossopriza lyoni (Blackwall)]. Depending at low temperatures that are otherwise suitable for growth on the group or species and on its particular life cycle, and development. other instars are modified. For example, the pre-adult Dormancies and related responses can greatly prolong (second nymphal) instar in several Acaridiae is the life-cycle durations. Many life-cycle programmes are hypopus, a modified resting instar. structured by diapauses that suppress larval development The number of instars typically is large in apterygotes in response to environmental cues of various kinds (e.g. up to 30 in silverfish: Nishizuka et al., 1998) and (Danks, 1987). Beyond such clearly marked develop primitive exopterygotes. Mayflies normally have 15-25 mental delays, the rate of growth can be modified indi instars, but from as few as 10 to as many as 52 depending rectly by photoperiod (Danks, 1987, Table 3); and simple on the species (Trost & Berner, 1963; Clifford et al., quiescence can be caused directly by unsuitable food or 1979; Fink, 1980; Brittain, 1982). There are about 12-24 by conditions that are too cold or too dry (see Delays). instars in stoneflies (Vaught & Stewart, 1974; Baumann, The possibility of such interruptions in the life cycle 1987), and 10-15 in dragonflies (Westfall, 1987). On the prompts care in developing experimental protocols to other hand, usually there are only five instars in Heterop- measure life-cycle duration. tera. Advanced endopterygotes typically have only 3-4 Once feeding ends, the final-instar larva enters the pre instars (many Coleoptera and most Diptera) or 4-7 instars pupal stage (Fig. 1). A few authors have restricted the use (most Lepidoptera and Hymenoptera). Although in many of this term, like the term propupa for Thysanoptera, to a species, especially of endopterygotes, the number of separate preadult instar. The end of feeding can some instars is fixed, in some species the number can vary times be detected by direct observation, although this is depending on the conditions of temperature and food often inadequate for rapid checking during experiments. experienced. For example, variations in the number of It is also signalled by changes in behaviour or by changes instars are known in spiders (Miyashita, 1997), ticks in appearance. For example, fully grown larvae may spin (Dautel & Knülle, 1997), mayflies (Brittain, 1976), stone- a cocoon, or leave the food to wander in search of a pupa- 286 Immature stages Adult stages opposition éclosion hardening mating oviposition en d o f d e a th oviposition (dispersal) te n e ra l i 50% ovposition prem a tin g ^ ______ ,_______tjl}___ ^ 4— development —► «-postoviposition* Fig. 2. Scheme showing visible stages of development in a typical oviparous insect, emphasizing adult stages.