Trophic determinants of hypopus induction in the stored-product mite Lepidoglyphus destructor (Acari: Astigmata).

A unique (synapomorphic) characteristic of astigmatic mites is the heteromorphic deuteronymph also called hypopus. It is a non-feeding and facultative instar between protonymph and tritonymph. The hypopus is adapted for dispersal and sometimes also for dormancy, as in Lepidoglyphus destructor. The experiments reveal a correlation between the composition of the foodstuff, the duration of development of homomorphic instars, the mortality of protonymphs and the production of hypopodes. As food quality decreases, development lasts longer, mortality increases and hypopodes are produced in greater numbers. Disadvantageous trophic conditions of varied chemical nature favour the induction of hypopodes. The experimental data show that hypopus incidences (as percentage individuals of a population) depend on the relative proportions of constituents of an ingested foodstuff. What matters is the ratio between nourishing foodstuff components and those that are of little or no nutritional value. When a certain ratio does not meet a presumed metabolically required level of nutrients a nutritional deficiency results and hypopus induction is triggered, provided that adequate genetic propensities for hypopus production are present (L. destructor is highly polymorphic for hypopus production). Specific key substances are apparently not involved, and composite properties of a foodstuff are crucial for hypopus induction. Decrease of food quality (not poor food per se) during the hypopus-inducible period (late larval to early protonymphal phase) promotes hypopus induction. The interpretation matches the ecological scene. When trophic deterioration of a patch habitat sets in, often as a result of overcrowding, conditions will eventually become untenable. As a response to incurring nutritional deficiencies the mites will induce hypopodes, which provide for escape from or survival at the decaying habitat patch. Experiments support the threshold model of quantitative genetics for hypopus expression as previously inferred from other experiments with L. destructor.

Seasonal oviposition and temperature requirements of eggs may limit northern distribution of European Argas reflexus (Acari: Argasidae).

Seasonal reproduction and embryonic development was investigated in Argas reflexus (F.) throughout a 5-yr period. Ticks were kept in desiccators at approximately 76% RH at 2 experimental sites--an attic and an outdoor aviary--that provided a natural photoperiod and temperature. Tick oviposition was limited to the summer months (June through August) regardless of the year of investigation and of the seasonal time of tick feeding. Engorged female ticks entered diapause between late July and late August. Ticks that had already started oviposition, stopped at that time and continued in the following year without requiring another blood meal. In the laboratory, A. reflexus eggs exhibited an upper lethal temperature (ULT50: 1-h exposure) of 44.1 degrees C (95% CL: 43.7-45.5 degrees C), which is in accordance with the increased egg mortality observed in the attic, where temperatures reached > or = 45 degrees C. A. reflexus eggs showed a low level of cold tolerance. Despite a supercooling point of approximately -25 degrees C, their lower lethal temperature (LLT50: 24-h exposure) was only -12.3 degrees C. Even at a typical winter temperature of 3 degrees C and 76% RH, 50% egg mortality occurred after only approximately 34 d. In the attic and the aviary, none of the A. reflexus eggs were able to overwinter successfully during any of the investigated winters, including comparatively mild ones. The results strongly suggest that the temperature sum available for embryonic development in summer/autumn limits the northern distribution of A. reflexus.

The influence of physiological age of Argas reflexus larvae (Acari: Argasidae) and of temperature and photoperiod on induction and duration of diapause.

The occurrence of diapause and quiescence was investigated in Argas reflexus engorged larvae, nymphs I and nymphs II. For diapause experiments, larvae were maintained at five different locations: at constant 20°C long day (LD; 17 h light:7 h dark) or short day (SD; 10 h light:14 h dark), at two locations with natural photoperiod and temperature and at one location with natural photoperiod but constant 15°C. At 20°C, diapause incidence was low in physiologically young larvae, increased with larval age, and then decreased to zero in specimens of increased physiological age. This pattern, observed both at constant LD and SD, suggests that the propensity to diapause changes with the physiological age of the unfed larva. The duration of diapause decreased with increasing larval physiological age at all locations, resulting in a seasonally synchronized moulting pattern. The results suggest that A. reflexus larvae are photoperiodically sensitive both before and after feeding and that decreasing daylengths may be particularly strong inductive stimuli. The developmental zero and thermal constant of the larvae were determined as 13.24°C and 220 degree-days, respectively. Degree-day measurements revealed that larval A. reflexus may enter a diapause of different length when fed between August and December and kept at natural daylength. Development of engorged nymphs I and nymphs II, but not of larvae, was ultimatively restricted at a temperature of 37.5°C, but immediately resumed at 25°C, demonstrating the occurrence of quiescence at high temperatures. Similarly, at a low temperature of 15°C, many nymphs I and II did not develop within 58 months, but did so successfully after transfer to 25°C, without additional food intake.

Cold hardiness, supercooling ability and causes of low-temperature mortality in the soft tick, Argas reflexus, and the hard tick, Ixodes ricinus (Acari: Ixodoidea) from Central Europe.

Seasonal supercooling points (SCPs=temperature of crystallization) and cold hardiness were investigated in the indigenous hard tick, I. ricinus, and in A. reflexus, a soft tick introduced to Central Europe from the South. Both species proved to be freeze-susceptible as well as highly susceptible to inoculative freezing. None of the postembryonic developmental stages of either species showed any distinct seasonal pattern of SCP. Unexpectedly, the introduced A. reflexus exhibited a distinctly higher degree of cold hardiness in terms of lower lethal temperature (LT(50): 24h exposure) as well as lethal time (T(50): time of survival at -10.1 degrees C) than I. ricinus. Engorged I. ricinus larvae as well as engorged summer acclimatized A. reflexus larvae showed some mortality at temperatures well above the SCP. This mortality was generally expressed as a failure of the following stage to eclose properly. A 10-day cold acclimation at +3 degrees C eliminated that kind of mortality in summer acclimatized A. reflexus larvae, but not in I. ricinus larvae. It was frequently observed that freezing of ticks resulted-possibly via leakage from the midgut-in a subsequent reddish brown discoloration of the ticks after thawing. Taking into account that discoloration was an indication of previous freezing, it was concluded, that after long-term exposure (for >/=30 days) at -10.1 degrees C, a temperature well above the SCP, some tick mortality could be observed that was not caused by previous freezing. Weighing experiments clearly demonstrated, that the level of dehydration was not critical for survival of A. reflexus during long-term cold exposure, even at low RH. This indicates, that cold-related factors other than freezing and dehydration were detrimental to this species.

The supercooling ability of ticks (Acari, Ixodoidea).

The supercooling capacity of nine laboratory-held species of ticks originating from different geographical areas, as well as five field-collected species from Germany, was investigated. All but one tick species showed mean supercooling points between about -17 and -23 degrees C, suggesting that the capacity to supercool to temperatures of < or = -17 degrees C might be an inherent property of many tick species unrelated to their geographic origin. Photoperiod did not influence the mean supercooling point in any of the species and there was also no distinct seasonal pattern of supercooling in seasonally acclimatized Dermacentor marginatus. Thus, the supercooling ability was independent of the presence/absence of diapause. The finding of thermal hysteresis in D. marginatus hemolymph raises the question of whether or not anti-freeze proteins are involved in the supercooling capacity of that species. An interspecies comparison revealed a weak negative correlation between relative water content and supercooling point of the ticks and an even weaker correlation between body mass or body water mass and the supercooling point. Since the ticks exhibited low supercooling points both before and shortly after feeding, the blood used as food should lack potent ice nucleators.

Genetic and environmental determinants of hypopus duration in the stored-product mite Lepidoglyphus destructor.

This paper reports a series of experiments over many years on hypopus duration and extends the preceding investigation (1987) on hypopus formation in Lepidoglyphus destructor (Schrank, 1781). The length of time required for hypopus physiogenesis (diapause development) is genetically programmed but influenced by environmental factors. This span of time is highly variable, and may extend from one week to more than a year. Spreading out the potential for hypopus completion over time is adaptive, since a pool of hypopodes with prolonged and staggered dormancies serves to spread the risk of emergence of tritonymphs over extended periods of time; it buffers the population against sudden drought to which all other stages of the life-cycle succumb. The additive structure and large variance of the genetic system underlying the length of time required for hypopus physiogenesis allows for the reconstitution of a broad spectrum of genotypes in every generation through the process of meiotic segregation and recombination during sexual reproduction. It favours stored variability, provides a 'fail-safe device' both for survival as well as development in irregularly fluctuating environments, and facilitates the adaptation of populations to local conditions. The trait for hypopus physiogenesis varies independently from that of hypopus formation, and is apparently free to adjust, without genetic constraints, towards an adaptive optimum. The response to selection is fast. Low environmental humidities and high temperatures accelerate physiogenesis of the hypopus. Completion of the hypopus stage and moulting to the tritonymph is triggered by high humidities at moderate temperatures. If environmental conditions preclude moulting, the hypopus following ending of physiogenesis enters a state of quiescence. In contrast the seasonal and largely predictably varying environments, in which essentially anticipatory and season-related token cues like photoperiod regulate the timing of so many arthropod life-cycles, L. destructor copes with sudden and fatal drought, as well as with unheralded and favourable humidities in its ephemeral habitats, mainly by excessive genetic polymorphism in hypopus duration and formation; some genotypes are always instantaneously fit to meet the respective environmental situation. The mite faces gradual food deterioration of its patchily distributed microhabitats by a short-term anticipatory and environmentally cued developmental switch mechanism, which lowers the threshold for hypopus induction. On top of genetic variability and phenotypic plasticity, any genotype X environment interaction provides for increasing flexibility above that from genetic polymorphism and environmental polyphenism alone. This extraordinary measure of adaptedness fits L. destructor for life in irregularly fluctuating environments.

Gross morphological changes in the salivary glands of Ixodes ricinus (Acari, Ixodidae) between bloodmeals in relation to active uptake of atmospheric water vapour.

The gross morphological changes in the salivary glands of Ixodes ricinus (L.) were investigated at the light microscopic level in various phases off the host with emphasis on the engorged nymph, in order to relate the capability of active vapour uptake in the course of postembryonal development to degeneration and regeneration of salivary-gland alveoli. Agranular alveoli in engorged immatures of I. ricinus, from detachment to the following early pharate phase, do not appear different from those of the unfed instars. This is also true for the female up to approximately the end of oviposition. During moulting, the agranular alveoli of the immatures degenerate and new ones are formed which are apparently already functional in teneral nymphs and adults. In contrast, granular alveoli, much enlarged in freshly detached immature I. ricinus, shrivel in the early post-repletion period and soon reach a highly reduced state which is maintained until apolysis. Subsequently, they disintegrate completely. The finding that engorged and detached immatures of I. ricinus with markedly atrophied granular alveoli are capable of active vapour uptake until some days after initiation of apolysis suggests that only agranular alveoli are responsible for producing the primary secretion involved in vapour uptake.

Prevalence of Borrelia burgdorferi in Ixodes ricinus ticks in Berlin (West).

In 1986, 1711 nymphal and adult Ixodes ricinus orginating from Berlin (West) forests were examined individually or in pools of up to 10 ticks for the presence of Borrelia burgdorferi, the etiologic agent of Lyme borreliosis. Detection of borreliae was carried out by means of a culture method using modified Barbour-Stoenner-Kelly-Medium (BSK II). Tick populations from 14 out of 15 locations contained positive specimens. The calculated minimal infection rate of pooled ticks was 2.5% in nymphs (n = 1365), 10.2% in females (n = 59), and 5.3% in males (n = 114). Among those ticks examined individually, none of the nymphs (n = 49) proved to be positive but B. burgdorferi was isolated from 8.2% of the females (n = 73) and 7.8% of the males (n = 51). Fifty-five out of 56 isolates were identified as B. burgdorferi by means of an indirect immunofluorescence test (IFT) using monoclonal antibody H 5332. From these results B. burgdorferi must be considered as being present in the Berlin area.

Water vapour uptake from subsaturated atmospheres by engorged immature ixodid ticks.

Contrary to current opinion, fully engorged and detached larvae and nymphs of some ixodid ticks consistently take up substantial amounts of atmospheric water vapour and thereby display their regulative capacity for maintaining water balance in subsaturated air. Net uptake of vapour generally begins some days after detachment and the capability persists until shortly after initiation of apolysis, a period which in diapausing specimens may extend up to several months. This was shown for Ixodes ricinus, Haemaphysalis punctata, and the North American I. dammini. Apparently, some other engorged ixodid immatures fail to exhibit net vapour uptake, as was shown for both larvae and nymphs of Dermacentor marginatus and nymphs of Hyalomma anatolicum excavatum. But there is some evidence for engorged nymphs of D. marginatus that active uptake of vapour does occur, masked by spiracular transpiration. Net uptake of vapour is apparently not possible during the pharate phases. In I. ricinus both teneral nymphs and adults are capable of achieving net water gains by active vapour uptake on the first day following ecdysis. There is new evidence from fully engorged I. ricinus immatures for the decisive role of agranular alveoli in the production of the salivary secretion involved in vapour uptake.

Genetic variability and ecological adaptability of hypopus formation in a stored product mite.

Occurrence in Lepidoglyphus destructor of a facultative developmental stage (hypopus) adapted for dispersal and dormancy depends on genotype X environment interaction. Dietary factors affect hypopus formation, but the response of individuals to food quality greatly differs according to genotype. Large genetic variation in the control of hypopus formation exists within as well as between populations. Response to selection is rapid and reversible. The trait has substantial potential for selective adaptation to unpredictably varying environments and for evolutionary change.

Water vapour uptake from the atmosphere and critical equilibrium humidity of a feather mite.

The feather mite Proctophyllodes troncatus Robin takes up water vapour from subsaturated atmospheres down to relative humidities between 55% and 60%. Vapour uptake increases with rising humidity of the surrounding air and the time to attain maximum water gain decreases.

Ultrastructure of the rectal sac, the site of water vapour uptake from the atmosphere in larvae of the oriental rat flea Xenopsylla cheopis.

A portion of the hindgut of Xenopsylla cheopis larvae, the rectal sac, is extraordinarily enlarged and differentiated into a specialized organ displaying well-elaborated transporting epithelia. This organ is unique among insect recta because of gutter-like arrangement of two ultrastructurally distinct cell types with an asymmetrical position of the membranemitochondria complexes. The cells of the dorsal gutter have highly folded apical plasma membranes on the lumen side. The cells of the ventral gutter show deep infoldings of the basal plasma membrane on the haemolymph side. Less elaborate folds originate from the opposite sides of the cells. The nuclei of the dorsal epithelial cells are located in the basal part; those of the ventral cells in the apical portion of the cell. The ultrastructural features of the rectal sac, its ventilation synchronized with rhythmic opening of the anus during phases of active water vapour uptake, as well as the reversible arrest of uptake by experimental occlusion of the anus, reveal that condensation and uptake of atmospheric water take place in this organ.