That carrot mite be past its best

If you are a regular here, you'll have noticed that I've been peering at the very small recently, and today is no exception. While sorting through our larder, I found a rather mouldy and sad-looking carrot hiding at the bottom of the veg basket. Normally it would be dsetined immediately for the compost, but the mould was big and fluffy, and you never know what might lie within...

Mould on an old and shrivelled carrot (yes, it is a carrot...)

Close up the mould consists of white and green-grey fibres with clumps of spores - but note the small invertebrates, the largest being around 0.5mm long

Zooming in, the fungal fibres/hyphae are clearer, and the invertebrates are 'mould mites' of the genus Tyrophagus

Although I don't intend to look at the fungus in too much detail, a simple slide preparation shows the typical fan-like arrangement of spore-bearing 'conidiophores' at the end of the stalks - either Penicillium or Aspergillus (something I wrote about here albeit on a rather different substrate).

Tiny greenish spores of Pencillium or Aspergillus on branched conidiophores (x400)

The mites are typical of Tyrophagus 'mould mites' (probably T. putrescentiae, possibly T. longior) which feed on moulds - as you can see, they are pale and shiny (not unlike animated jelly-beans) with long sparse sensory hairs. They can be associated with damp conditions and respiratory or skin ailments (e.g. where houses have large damp, mouldy patches), although in this case our larder is dry and I expect they came in with soil, so hopefully throwing the carrot out and cleaning the basket will get rid of them! Although mites are generally familiar as a broad group, their finer structure tends to be less familiar unless you are reader of acarology (mite-science) webpages and books, so, zooming in will hopefully provide some useful insights.

Dorsal view of Tyrophagus, approx 0.5mm long. Note the long hairs, eight legs, and triangular arrangement of mouthparts.

Ventral view of Tyrophagus. Some internal structures are visible such as the genital area (central orange-brown patch), ovary (smaller orange-brown patch to the right), reproductive tract (dark patches either side of the ovary) and eggs (the oval structures towards the outside). The other blurry dark forms are branches of the gut.

A cleared specimen of Tyrophagus. The pinkish leg segments are clearly visible (as are the mouthparts to a lesser extent) and the orange ovary is also obvious.

The mouthparts are also worth investigating - being arachnids, mites have paired jaws (chelicerae) with a small fang.

Ventral view of mouthparts of Tyrophagus. The paired jaws (chelicerae) can be seen, each ending in a small fang. The pharynx is just visible as a faint elongate structure running off to the right. Note the mould spores stuck to the mite (here I speculate that as well as wind-dispersal, the spores may be spread by mite movements).

Dorsal view of the rostrum of Tyrophagus, above the chelicerae. Sensory features are also visible here, including socketed setae (green arrow) and what appear to be ocelli (red arrows).

Colloff (2009) shows how little is known about the sensory apparatus of mites - they certainly don't have eyes that can form an image but which have simple ocelli as photoreceptors is unclear. I can not find Tyrophagus listed as having ocelli, but while Woolley (1988) only mentions the family Saproglyphidae as having them, this specimen does appear to (and is in the Acaridae). However, the long setae are more important sensory structures. The socket is a membranous receptor and while they undoubtedly have a mechanical function (e.g. sensing touch, vibration or similar), at least some have pores with research suggesting (though a little tentatively) that they may also sense chemicals such as pheromones (Leal & Mochizuki, 1990)

Seta on the dorsal body surface of Tyrophagus, again showing the socket.

Moving on from sensory structures to locomotion, the leg segments, including inter-segmental membranes, are clearly visible in some of the previous photos. However the 'foot' (i.e. the tip of the last tarsal segment) has some interesting structural features.

Tip of the last tarsal segment of Tyrophagus. Note the flanged expansion and single small claw (curving towards the camera here).

Lastly, I want to revisit the egg seen inside the whole mite in one of the first photos above; note that mating is rear-to-rear as seen in Fig. 1c here as well as below) and the male is noticeably smaller than the female and doesn't have the orange ovary.

Mating pair of Tyrophagus

Large, well-developed egg of Tyrophagus removed from a female. This appears sculptured whereas the few available images of eggs are smooth apart from microsculpturing. Therefore, this may have a covering that is lost upon laying.

 That is where I'd like to leave mite morphology/anatomy, for now at least, although I suspect they'll appear on my radar again at some point.

Even moulds can be attractive - a spore-covered pom-pom!

References

Colloff, M.J. (2009). Dust Mites. CSIRO, Highett, VIC.

Leal, W.S. & Mochizuki, F. (1990). Chemoreception in astigmatid mites. Naturwissenschaften 77: 593-594.
Woolley, T. (1988). Acarology: Mites and Human Welfare. Wiley, New York.