Abstract
Mid- and hind-gut lumenal changes are described in the free-living predatory soil mite Pergamasus longicornis (Berlese) from a time series of histological sections scored during and after feeding on fly larval prey. Three distinct types of tangible material are found in the lumen. Bayesian estimation of the change points in the states of the gut lumenal contents over time is made using a time-homogenous first order Markov model. Exponential processes within the gut exhibit ‘stiff’ dynamics. A lumen is present throughout the midgut from 5 min after the start of feeding as the gut rapidly expands. It peaks at about 21.5 h–1.5 days and persists post-feeding (even when the gut is contracted) up until fasting/starvation commences 10 days post start of feeding. The disappearance of the lumen commences 144 h after the start of feeding. Complete disappearance of the gut lumen may take 5–9 weeks from feeding commencing. Clear watery prey material arrives up to 10 min from the start of feeding, driving gut lumen expansion. Intracellular digestion triggered by maximum gut expansion is indicated. Detectable granular prey material appears in the lumen during the concentrative phase of coxal droplet production and, despite a noticeable collapse around 12 h, lasts in part for 52.5 h. Posterior midgut regions differ slightly from anterior regions in their main prey food dynamics being somewhat faster in processing yet being slightly delayed. Posterior regions are confirmed as Last-In-Last-Out depots, anterior regions confirmed as First-In-First-Out conveyor belt processes. Evidence for differential lability of prey fractions is found. A scheme is presented of granular imbibed prey material being first initially rapidly absorbed (t_{frac{1}{2}} = 23 min), and also being quickly partly converted to globular material extra-corporeally/extracellularly (t_{frac{1}{2}} = 36 min)—which then rapidly disappears (t_{frac{1}{2}} = 1.1 h, from a peak around 4 h). This is then followed by slow intracellular digestion (t_{frac{1}{2}} = 6.9 h) of the resultant resistant prey residue matching the slow rate of appearance of opaque pre-excretory egestive refractive grains (overall t_{frac{1}{2}} = 4.5 days). The latter confirmed latent ‘catabolic fraction’ (along with Malpighian tubule produced guanine crystals) drives rectal vesicle expansion as ‘faeces’ during the later phases of gut emptying/contraction. Catabolic half-lives are of the order of 6.3–7.8 h. Membraneous material is only present in the lumen of the gut in starving mites. No obvious peritrophic membrane was observed. The total feeding cycle time may be slightly over 52.5 h. Full clearance in the gut system of a single meal including egestive and excretory products may take up to 3 weeks. Independent corroborative photographs are included and with posterior predictive densities confirm the physiological sequence of ingestion/digestion, egestion, excretion, defecation, together with their timings. Visually dark midguts almost certainly indicate egestive refractive grains (xanthine?) production. Nomograms to diagnose the feeding state of P. longicornis in field samples are presented and show that the timing of these four phases in the wild could be inferred by scoring 10–12 mites out of a sample of 20. Suggestions to critically confirm or refute the conclusions are included.
Highlights
There is the opportunity in any animal that ingests food, for digestion to occur within the gut before nutrient assimilation
Bowman (2017) in following the time for a pulse of larval dipteran prey to be fully processed in P. longicornis into Malpighian waste guanine crystals, points out that visible gut expansion/contraction is not a comprehensive measure of all nutritional processes
Bowman (2017) pointed out that an understanding of the time course of the production of any waste material in the gut lumen would help confirm or refute explanations of pergamasid digestive mechanisms offered. Further he described potential evidence for a hidden ‘egestive fraction’ of partly assimilated-partly catabolised material driving mesostigmatid rectal vesicle expansion. Irrespective of whether this arises from intra- or extracellular digestion, such pre-excretory material ought to be detectable in the gut lumen during feeding on prey by P. longicornis if systematically looked for
Summary
There is the opportunity in any animal that ingests food, for digestion to occur within the gut before nutrient assimilation. Using gut expansion/contraction measures, Bowman (2014) better estimated total feeding and digestion time in this poikilotherm in the laboratory as 52.5 h These range of figures agree with the complete disappearance of a meal in 3–7 days found on examination of a slightly smaller mesostigmatid Echinolaelaps echidninus by Kanungo (1964). Bowman (2017) pointed out that an understanding of the time course of the production of any waste material in the gut lumen would help confirm or refute explanations of pergamasid digestive mechanisms offered Further he described potential evidence for a hidden ‘egestive fraction’ of partly assimilated-partly catabolised material driving mesostigmatid rectal vesicle expansion. Irrespective of whether this arises from intra- or extracellular digestion, such pre-excretory material ought to be detectable in the gut lumen during feeding on prey by P. longicornis if systematically looked for
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