Development of respiratory structures in embryos and first and second instars of the bark scorpion,Centruroides gracilis (Scorpiones: Buthidae)

The SEM was used to study the development of respiratory structures in successive stages in relation to the overall changes occurring in the scorpions. Book lung development is a slow process, starting with spiracles and a sac‐like atrium in the early embryo and continuing lamellar formation to 150 or more in the adult. In the embryo, the primordial epithelial cells become aligned in a planar pattern as they secrete granules of material that aggregate spontaneously to form the cuticular walls of the lamellae. A blade‐like structure is formed consisting of cells sandwiched within the two cuticle walls they secreted. These cells are in the primordial air channel. The adjacent hemolymph channel is nearly devoid of cells, but cross‐bridges develop and help stabilize the cuticle walls and maintain the width of the channel. The cells in the primordial air channel undergo cytolysis, leaving it open for air except for cuticular cross‐bridges. Development continues in the newborn (first instars); the air channels of some lamellae still contain cells and are not yet functional for gas exchange. The first instars are weak and relatively inactive. They climb up on the mother's dorsum until the first molt (about 8 days). With the cuticular walls of the lamellae in place, cells adhering to the wall in the hemolymph channel produce a thin, new tissue layer (epithelium) on the lamellar wall facing the hemolymph channel. This layer has many discontinuities as though it is slowly developing. Formation of the tissue layer and cytolysis of the cells in the air channels continue through the first molt in which little book lung cuticle is shed as exuvium. The air channels of the second instars (foraging nymphs) are now cell free and open for air passage except for the cross‐bridges. The tissue layer is still incomplete and continues to be formed. It may provide the hypodermal primordium for cuticle replacement in later molts, but development was not studied beyond the second instar except for comparison with book lungs in the adult. The blade‐like lamellae in the adult are larger and more numerous than in the second instar, but in the anterior book lung the shape of the cuticle wall and cross‐bridges and the widths of the air and hemolymph channels are about the same as in the second instar. The air channels in the posterior part of the lamellae have distinctive, vein‐like space‐holders. The similarity of the adult anterior lamellae with those in the second instar suggests retention of this part through the 4–5 molts to maturation, and/or cell processes like those in the embryo are repeated, but this needs to be examined in further studies of cell and cuticle changes before and during the molts. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Developmental changes in the embryo, pronymph, and first molt of the scorpion Centruroides vittatus (scorpiones: buthidae)

For the first time the scanning electron microscope was used to compare developmental changes in scorpion embryos and the first and second stadia. In the buthid species of this study, Centruroides vittatus, and all other scorpions, the newborn climb up on their mother's back and remain there without feeding for several days. At this location, they undergo their first molt and in a few days they disperse, fully capable of foraging in the terrestrial environment. The results here support earlier suggestions that the first stadium (pronymph) is a continuation and extension of embryological development. The first molt results in a nymph with exoskeletal features much like those in the adult. In the first molt the metasoma becomes relatively longer, and the sting (aculeus) becomes sharp and functional. The metasomal segments are modified for dorsal flexion and sting use. The embryos and the pronymphs have spiracles that open into an invagination near the posterior margin of flap-like abdominal plates in segments 4-7 of the ventral mesosoma. The second instars have spiracles that lead to book lungs farther anterior in sternites. Tubular legs with cylindrical segments in embryos and pronymphs become more sculptured and oval in the transverse plane. Each leg in the pronymph has a blunt, cup-shaped tip while distal claws (ungues, dactyl) are present in the second instar and subsequent stages. There are some sharp bristles and primordial sensilla in the pronymphs, but the second stadium has adult-like surface features: rows of knobs or granulations (carinae), serrations on the inner surfaces of cheliceral and pedipalpal claws, filtering hairs at the mouthparts, peg sensilla on the pectines, and mechano- and chemoreceptor sensilla on the body and appendages. Scorpion embryos and pronymphs have some structures like fossil scorpions thought to have been aquatic. There is a gradual development of features that appear to be terrestrial adaptations. Evidence is provided for the formation of the sternum from third and fourth leg coxal primordia and possibly from the first abdominal segment. This study is the first to provide evidence for a forward shift of the gonopore along with other structures in the anterior abdomen.     

Decreased JH biosynthesis is related to precocious metamorphosis in recessive trimolter (rt) mutants of the silkworm, Bombyx mori

In recessive trimolter (rt) mutants of the silkworm, Bombyx mori, that have four larval instars rather than five larval instars of normal B. mori, a decrease after a small increase in the hemolymph ecdysteroid titer during the early stages of the last (fourth) larval instar appeared to be a prerequisite for larvae to undergo precocious metamorphosis. The present study was carried out to investigate the possible mechanism underlying this decrease in the ecdysteroid titer. It was found that juvenile hormone (JH) biosynthetic activity of the corpora allata (CA) increased during the first day of the last larval instar, but its absolute JH biosynthesis activity was relatively lower compared to that of normal fourth-instar larvae in tetramolters. This lowered JH biosynthetic activity appeared to be related to a decrease in prothoracic gland ecdysteroidogenesis during the second day of the last instar, because hydroprene application prevented this decrease in prothoracic gland ecdysteroidogenesis, leading to the induction of a supernumerary larval molt. The in vitro incubation of prothoracic glands with hydroprene showed that hydroprene did not directly exert its action on prothoracicotropic hormone (PTTH) release. Further study showed that the application of hydroprene enhanced the competency of the glands to respond to PTTH. From these results, it was supposed that the lowered JH biosynthesis of the CA during the first day of last instar in rt mutants was related to decreased ecdysteroidogenesis in the prothoracic glands during the second day, thus playing a role in leading to precocious metamorphosis.     

HISTORICAL EXAMINATION OF DELAYED PLUMAGE MATURATION IN THE SHOREBIRDS (AVES: CHARADRIIFORMES)

Delayed plumage maturation refers to the presence of nonadultlike immature plumages (juvenal plumage excluded). It is usually considered the result of selection for distinctive first-winter or first-summer appearance. In the present study, evolution of delayed plumage maturation is examined in the shorebirds: the sandpipers, plovers, gulls, and their allies. Nine plumage-maturation characters were identified, and their states were superimposed onto topologies generated during two recent investigations of shorebird relationships (Sibley and Ahlquist; revised Strauch). The characters were then optimized so as to assign character states to interior nodes of the trees in the most parsimonious way. Reconstructions of character evolution on six of the shortest revised Strauch trees were ambiguous with respect to delayed plumage maturation in the hypothetical ancestral shorebird. If plumage maturation was not delayed in the shorebird ancestor, optimization indicated that delay appeared when nonadultlike juvenal feathers were acquired. In contrast, on the single Sibley and Ahlquist tree, absence of delayed plumage maturation in the shorebird ancestor was indicated unambiguously, with three evolutionary novelties (nonadultlike juvenal feathers, seasonal plumage change, and a reduced first-spring molt) implicated in its acquisition. Optimization indicated that delayed plumage maturation in shorebirds can be explained plausibly without invoking selection for distinctive first-winter or first-summer appearance. Two of the novel conditions generating delayed plumage maturation (modified juvenal feathers and seasonal plumage change) did so only because they were acquired in a taxon possessing restricted first-year molts, which are primitive. Given these observations, it seems simplest to explain the delay in plumage maturation as an incidental consequence of the phylogenetic inertia of shorebird molts. The third novelty that generates delayed plumage maturation, a reduced first-spring molt, may have been acquired to reduce molt-associated energetic demands in young birds.     

Within-individual changes in developmental stability affect flight performance

Developmental stability, as measured by fluctuating asymmetry,has been purported to be an indicator of individual quality,and low asymmetry can be selected for by sexual selection processes.However, low asymmetry can also arise due to biomechanical constraintsoperating on trait design, as it is predicted that asymmetrywill decrease mechanical efficiency. Specifically, it has beenpredicted that wing length asymmetry will be negatively relatedto avian flight performance. To date, empirical investigationshave only studied the influence of increasing asymmetry beyondnaturally occurring average values. I examined the influenceof within-individual changes in primary feather developmentalstability on flight performance in European starlings by studyingasymmetry and flight before and after wing molt. Individualsthat exhibited a decrease in wing asymmetry through molt experiencedincreased aerodynamic performance in terms of both angle oftakeoff and level flapping-flight speed. Birds that increasedwing asymmetry suffered a decrease in flight performance. Takeoffspeed and the ability to negotiate an aerial obstacle coursewere unaffected by asymmetry. My data provide empirical supportfor the predicted influence of wing asymmetry on flight, eventhough the changes in asymmetry were very small (mean = 0.47%of trait size) and further indicate the importance of biomechanicalconsiderations in any study of developmental stability     

Monitoring of rice weevil, Sitophilus oryzae, feeding behavior in maize seeds and the occurence of supernumerary molts in low humidity conditions

Rice weevils, Sitophilus oryzae (L.), complete their development from egg to early adulthood inside grains of wheat, rice or maize, but little is known about their feeding behavior within the seeds. An ultrasonic insect feeding monitor was used to characterize the feeding patterns of individual rice weevils as they developed in maize grains. Weevils reared in grains held at ca. 40% r.h. took longer to develop from egg to pupation than those reared in grains kept at ca. 70% r.h. Feeding patterns revealed that larvae developing in grains held at ca. 70% r.h. had only four instars, whereas supernumerary molts (five instars) occurred in certain individuals reared in grain held at ca. 40% r.h. The supernumerary molts may represent a response of the insect to stresses associated with low moisture levels in the seeds.     

Local weather and body condition influence habitat use and movements on land of molting female southern elephant seals (Mirounga leonina)

Southern elephant seals (Mirounga leonina) are known to move and aggregate while molting, but little is known about their behavior on land during this time. In this study, 60 adult females were monitored (23 with GPS tags) during four molting seasons, between 2012 and 2016 at Kerguelen Archipelago, Indian Ocean. Population surveys were recorded each year (N = 230 daily counts), and habitat use was analyzed in relation to the stage of the molt and local weather. Based on stage of molt, habitat use, and movements on land, we classified the molt of elephant seals into three phases: (1) a “search phase” at the initial stage of molt when grass and wallow habitats were used and characterized by greater mean distances travelled on land per day compared with the two other phases; (2) a “resident phase”: during initial and mid‐stage of molt when animals were found in grass and wallow habitats but with less distance moved on land; and (3) a “termination phase” at the final stage of molt where grass and beach habitats were occupied with no change in distances. Windchill and solar radiation influenced individual distances moved per day (mean 590 ± 237.0 m) at the mid‐ and final stage of molt such that animals travelled greater distances on days of low windchill or high solar radiation. Individual variation in distance moved and relative habitat use were also linked to body mass index (BMI) at arrival on the colony, as females with higher BMI moved less and preferred beach habitat. Moreover, the individual rate of molt increased with the use of wallows. Aggregation rate tended to be negatively correlated with distances moved. We therefore suggest that individuals face an energetic trade‐off while molting, balancing energy expenditure between movement and thermoregulation.     

The trade-off between molt and parental care: a sexual conflict in the blue tit?

Breeding activities and molt are generally thought to be mutuallyexclusive in birds since both are energetically costly and arenormally separated in time. However, sometimes molt overlapswith breeding to some degree. A trade-off between adult somaticmaintenance functions (feather renewal) and parental care isthen to be expected. The consequences of this are largely unknown,and there are few studies that have shown any fitness costsof molt-breeding overlap. We investigated the consequences ofmolt-breeding overlap by removing first clutches of blue titParus casruleus pairs, thereby inducing late repeat clutches.Among the delayed pairs, a high proportion of males and somefemales started their molt already during incubation or nestlingfeeding. Molting males fed their nestlings to a lesser extentthan non-molting ones, and nestling mortality increased as adirect result of the early timing of male molt. Furthermore,the ability to raise an experimentally enlarged brood was negativelycoupled to the molt stage of the male. Our data thus provideevidence that molt-breeding overlap leads to fitness costs,and we discuss the results within the context of sexual conflictand the implications for optimization of avian reproductivedecisions