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ŠUMARSKI LIST 11-12/2016 str. 25     <-- 25 -->        PDF

inside their substrates. But with increasing numbers of mites after about two weeks, mites began to develop in all kinds of areas, as far as the habitats were not too dry or too moist. They preferred a substrate being covered by a thin slimy film of moisture.
The gamasid mite Dendrolaelaps quadrisetus was observed being often actively walking around areas, where Histiostoma piceae specimens appeared in greater numbers. The same areas were also colonized by free living nematodes. It is not known, whether D. quadrisetus was feeding on histiostomatids, nematods, both of them or more likely on other organisms.
5.2.) fungus spores attached to Histiostoma sp.
Histiostoma sp. was observed being remarkably covered by substrate particles, different smaller fungal spores, but especially by conspicuous two-chambered spores (Fig. 6 E). They belong to an undetermined species of Ascomycota (Hypocreales). Single females of H. sp. carried 100 or more of these spores over their whole bodies. They especially were sticking in greater numbers to the dorsal area of the gnathosoma, to legs I and II and to the distinctly elongated setae of the hysterostoma.
The spores attached to the mite body due to the sticky cuticle, which is typical for astigmatid mites. Oil gland components are responsible for this effect (i.a. Koller et al. 2012). The spores additionally were sticking against each other due to an unknown mechanism. Also the confusing arranged dorsal setation of the mites allowed the mechanical holding of these spores and all other visible particles.
Mites were not visibly harmed by these objects, which they were carrying on their backs. They were motile and showed a healthy histiostomatid behavior.
6.) key to free living stages of Histiostoma bark beetle-group
The nomenclature of dorsal setae in larvae and protonymphs fellows Griffiths et al. (1990).
larva (histiostomatidae/ Astigmata):
1. mouthparts modified: digitus mobilis reduced to remnants, distal pedipalp article bulged sidewards, membraneous structures at distal pedipalps ---- 3.
2. mouthparts in the typical arachnid shape ---- 4.
3. larvae of the Histiostomatidae ---- 5.
4. larvae of other Astigmata groups
5. the following dorsal setae arranged on separate distinctly sclerotized cuticular plates: median plates containing a pair of setae: setae d1 and setae e1; single plates on each side for c2 and h2 ---- Histiostomatidae
cp on a rounded cuticular shield ---- Genus Histiostoma ----6.
cp on indistinct elevation or completely without cuticular shield ---- other Genera of Histiostomatidae
6. setae c3, c2 and c1 on a common cuticular plate on each side --- bark beetle clade within genus Histiostoma (Fig. 7 B)
protonymph (Histiostomatidae/ Astigmata)
1. mouthparts modified: digitus mobilis reduced to remnants, distal pedipalp article bulged sidewards, membraneous structures at distal pedipalps ---- 3.
2. mouthparts in the typical arachnid shape ---- 4.
3. protonymphs of the Histiostomatidae ---- 5.
4. protonymphs of other Astigmata groups
5. setae f2 and oilgland opening on each side arranged on a common cuticular plate, which can also be a partly sclerotized area, thus forming no complete rounded shield---- Histiostomatidae, different genera
setae d1 and e1 on two median cuticular shields containing setae of both sides --- bark beetle clade within genus Histiostoma (Fig. 7 A)
Comparisons between numbers and positions of phoretic mites on the beetles I. typographus and I. cembrae from Croatia were never performed before. Our analysis in this context was based on qualitative information as we had decided not to use traps, but to collect all beetles available for our evaluations alive. Thus we ensured that phoretic mites remained on their beetles in their orginal positions.
An interesting finding concerned comparisons between young beetles, emerged from their juvenile galleries, with older parental beetles. Numbers of attached mites on these beetle groups were statistically similar. Samplings on a random basis of larvae, pupae and freshly hatched beetles of I. cembrae had no phoretic mites. This was confirmed by a qualitative study on the closely related I. typographus (galleries collected in Jastrebarsko). According to these findings phoretic mites, in this case I. gaebleri and H. piceae, do not secure their carriers in the pupa stage or shortly after their hatching, but anytime subsequently. There is no indication about the detailed circumstances, in which I. gaebleri males get on their carriers. But deutonymphs of H. piceae might ascend their beetles in the areas of their exit holes. This assumption is supported by observations, which Wirth (oral communication) made in older studies on a related mite species from bark beetle galleries of an undetermined and absent beetle in Louisiana (USA). Deutonymphs here had conspicuously aggregated around the numerous exit holes.
Although especially Histiostoma piceae seemed to prefer areas close to living beetles for their development, they did not develop on or around beetle cadavers. This is confirmed by our findings, whereby both mite species, I. gaebleri and H. piceae, remained for a longer while on died beetles, but then left these cadavers consecutively. The strategy of necromeny (Wirth, 2009) is obviously not even practised as a byway by both mite species.
The finding that mites stay for days on the cadavers of their carriers indicates that the stimulus to leave depends much