Abstract

Arthropods require periodic molting in order to grow which presents a number of challenges to terrestrial taxa. Following ecdysis, the pliant new cuticle is susceptible to buckling under gravity and requires elevated hydrostatic pressure for support. Terrestrial species also require a mechanism of volume expansion and stretching of the integument prior to sclerotization, a need that is readily met in aquatic arthropods by drinking. Options for land arthropods include drinking of dew, swallowing of air, or using muscular contractions to inflate air sacs in tracheate taxa. In this study we tested the hypothesis that crinochete terrestrial isopods (Isopoda: Oniscidea: Crinocheta) exploit their capacity for active water vapor absorption (WVA) to increase volume during molting. Two crinochete species, Armadillidiumvulgare and Porcelliodilatatus, were studied and compared with the non-absorbing species Ligidiumlapetum (Oniscidea: Ligiamorpha). Pre-molting animals were identified by sternal CaCO3 deposits and exposed to 100% or 97% relative humidity (RH). Mass-changes were monitored by daily weighing and the timing of the posterior and anterior ecdyses was used to categorize time (days premolt and days post-molt) over the molt cycle. In each treatment RH, A.vulgare and P.dilatatus showed a progressive mass increase from 5 days premolt until the posterior or anterior ecdysis, followed abruptly by period of mass-loss lasting 3–4 days post-molt. The fact that the initial mass-gain is seen in 97 % RH, a humidity below the water activity of the hemolymph, confirms the role of WVA. Similarly, since the post-molt mass-loss is seen in 100 % RH, this must be due to active expulsion of water, possibly via maxillary urine. Concurrent changes in hemolymph osmolality were monitored in a separate batch of A.vulgare and show sustained osmolality during premolt and an abrupt decrease between the anterior and posterior ecdysis. These patterns indicate a mobilization of sequestered electrolytes during premolt, and a loss of electrolytes during the post-molt mass-loss, amounting to approximately 8.6 % of total hemolymph solutes. WVA, in conjunction with pulses of elevated hemolymph pressure, provides an efficient mechanism of pre-molt volume expansion prior to and during the biphasic molt in these species. Premolt Ligidiumlapetum exposed to same treatments failed to molt successfully and no premolt animals survived to day 3 (72 h) even in 100 % RH. The apparent dependence of this species on liquid water for successful molting could explain its obligatory association with riparian fringe habitats.

Highlights

  • The cuticle of arthropods is an organ of extraordinary adaptive versatility, allowing for articulation and movement via complex joints, sensory transduction using a remarkable variety of permeable or deformable sensilla, variable morphology and coloration from impregnated pigments or refractory laminae, and extreme resistance to water loss from intrinsic or superficial lipids in many terrestrial taxa (Locke 1965, 1974, Chapman 2012)

  • Armadillidium vulgare and Porcellio dilatatus were collected from the Pomona College campus and vicinity, Claremont, CA, and Ligidium lapetum was collected from local foothill canyons in the San Gabriel Mountains

  • Mass gain declined sharply or reversed 1–2 days after Posterior ecdysis (PE) and was followed by a period of water loss. This was quite variable among animals in 97 % relative humidity (RH), but more clearly defined in 100 % RH with the largest loss fluxes occurring in the day following anterior ecdysis (AE) and declining progressively thereafter

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Summary

Introduction

The cuticle of arthropods is an organ of extraordinary adaptive versatility, allowing for articulation and movement via complex joints, sensory transduction using a remarkable variety of permeable or deformable sensilla, variable morphology and coloration from impregnated pigments or refractory laminae (physical coloration), and extreme resistance to water loss from intrinsic or superficial lipids in many terrestrial taxa (Locke 1965, 1974, Chapman 2012). Subsequent activation of the molting fluid results in degradation and resorption of the old endocuticle (Samuels and Reynolds 1993). At this stage, the new cuticle is soft and flexible to allow for expansion and growth. Pliant intersegmental cuticle may continue to expand between molts, allowing for a sustained size increase, but mineralized or sclerotized cuticle does not expand. This was well illustrated by Clarke (1957) who showed that intermolt tibia length remains constant in Locusta, increases abruptly at ecdysis, despite a steady increase in whole-animal

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