PupaeAfter completing development, the late third instar larva voids its alimentary canal contents in preparation of forming a cocoon and moves to an undisturbed place to spin a silk-like cocoon in which it pupates (Lyons 1915; Karandikar and Munshi 1950; Joseph 1981). The principal factors triggering pupation are declining levels of juvenile insect hormone (Grant 1996). The silkin excrete for the cocoon is produced by the salivary glands (Strenger 1973). The resulting cocoon consists of soft and moist silk-like material (Dryden 1993), measures about 4 by 2 mm (Soulsby 1982), is loosely spun and whitish in color (Dryden 1993) and is coated with dust and debris because of its stickiness (Soulsby 1982) which aids in camouflaging it perfectly (Karandikar and Munshi 1950; Dryden 1989a). Flea cocoons can be found in soil, on vegetation, in carpets, under furniture, and on animal bedding (Dryden 1993) (Fig. 9).
Three stages within the pupal cocoonWithin the pupal cocoon three distinct stages are found in the order Siphonaptera (to which the cat and the dog flea belong): The first is the U-shaped larval prepupa, the second is a true exarate pupa and the third is the preemergent adult, which has completed its pupal-imaginal molt but remains within the cocoon for varying lengths of time (Silverman et al. 1981b) (see ‘Preemerged Adults’). The U-shaped larva begins pupal development about 18 hours after the completion of the cocoon (Dryden and Smith 1994). If the larva is disturbed prior to this time in form of gentle shifting of the larval medium (e.g.) it might emerge and exit the cocoon. In trials by Dryden and Smith (1994) over 40% of those larvae did not spin a second cocoon, but developed as naked flea pupae. Metzger and Rust (1997) observed shorter times for adults to emerge from naked pupae compared with emergence time from cocoons. The importance of this early emergence which can also be produced in carpeted homes by the vigorous agitation of the carpet by vacuums with rotary beater bars is so far unknown (Dryden and Smith 1994). Of importance for a successful formation of the cocoon is the orientation against a perpendicular structure. Less than 3% manage to spin an enclosing cocoon in the absence of a vertical surface, but over 95% of the larvae nevertheless survive to adulthood (Dryden and Smith 1994), thus demonstrating that the cocoon is not essential for the development of adults (Dryden and Smith 1994). Nevertheless it offers some benefits as e.g. protection from ant predation (Silverman and Appel 1984), somewhat impeding adult emergence and offering protection from non-host-produced stimuli and thus minimizing non-host-induced emergence (Silverman and Rust 1985). It does not provide a barrier to water loss (Silverman and Rust 1985) and also does not seem to be a barrier to insecticides (Dryden and Reid 1993), even though cocoons placed in a carpet appeared to be highly tolerant to a variety of insecticides (Rust and Reierson 1989). The survival of developing pupae in insecticide-treated households is caused by the lack of penetration of the carpet canopy by the insecticide (Dryden and Rust 1994). Prepupae and pupae are fairly resistant life stages (Silverman et al. 1981b), but it is unknown how much of that protection is provided by the cocoon (Dryden and Smith 1994). Favorable condition: 27°C and relative humidity >50%At 27+/-2°C, females pupate within about 32 hours and males pupate an average 12.1 hours later within about 44 hours (Dryden and Smith 1994). The degree of development within the cocoon at 16°C and 27°C was similar regardless of humidity (in trials of Silverman et al. 1981b). Under moderate conditions the pupa was the immature stage most resistant to desiccation; 80% survived to adulthood at 2% RH and 27°C (Silverman et al. 1981b). Exposure to 35°C during pupal development is uniformly lethal (Silverman et al. 1981b). Exposure to 3°C for five days and 8°C for 20 days is lethal as well (Silverman and Rust 1983). Females develop into adults about 1.6 days faster than males (Dryden and Smith 1994). In trials of Metzger and Rust (1997) male prepupae and pupae developed more slowly than females at each temperature, requiring 14-20% more time to develop. At 15.5°C, the mean number of days for the female and male pupae to develop was 19.5 (female) and 23.5 (male), respectively (Metzger 1995), or even nine days earlier (Metzger and Rust 1997) and at 26.7°C, the mean number of days was 5.2 (female) and 6.5 (male), respectively (Metzger 1995), or two to five days earlier (Metzger and Rust 1997). Not only females but also adults that developed from light prepupae in general remained in their cocoons for a shorter time than adults from more robust prepupae with two explanations proposed: either because of emergence mechanisms triggered when water and food reserves drop below a critical level or because of a weaker cocoon produced by incompletely nourished prepupa that does not impede adult emergence (Silverman and Rust 1985). When pupae were maintained at 24.4°C and 78% RH, adult C. felis began to emerge eight days after the initiation of pupal development and by day 13, all fleas emerged (Dryden 1988). The pupa is suggested to be the stage most likely to survive extended periods in cool dry climates (Silverman et al. 1981b). Summary see Box 3.
References
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