Velvet Mites, Trombidioidea and Chiggers

With this post I am going out on a limb. I got an e mail request to identify a small red bug. I am using bug here as a general term for arthropod, because:

Question:
'Hello Dr. Brummermann,

My co-worker and I were on Mt Lemmon today, in the Marshall Gulch area and we came across thousands of tiny red insects. Our first thought was chiggers, but after looking into it, these insects were too big to be chiggers.

They were bright red, had 6 legs, and large antennae. About 25% of the ones I looked at appeared to be dead (curled in on themselves and dry), but this could have just been the case for the one location that I was seriously investigating them in.
While I don't have any pictures, I can guess they were approximately the size of this "o" (San Serif, 12pt font). They were found in areas with large amounts of dead leaves, and when we dug under the leaves, they appeared to be moving through the soil.
Do you have any ideas as to what we saw?'

Answer:
Without a photo there is not too much to go on. What seemed to be dead ones could be exuviae - in that case I would guess that they were some early instar true bugs or other hemipterans. But since they seemed to be soil related and bright red, how about mites instead of insects? Their front legs could be mistaken for antennae. Here is a photo of a giant velvet mite: http://bugguide.net/node/view/200679
They are usually bigger than your 'o' but.... There are often outbreaks with thousands of them. They are predators as adults.
Let me know what you think and consider carrying a camera for documentation!

Trombidiidae (Velvet Mites) from my backyard. These live in sandy soils, are huge, around 5 mm in length and show up only after rains

Question:
Thank you very much for the speedy reply!

They could have been mites; we didn't have any magnifying lenses on us at the time, so I could have mistaken the front legs for large antennae. Will they bite humans (or other large mammals) or do they only go after other insects? If I'm up in the area again within the next few weeks, I'll take a picture!

Thank you again,

Answer:
At first, I was going to say
Your observation that the bugs were in the leaf litter makes me think that they are really in the family Velvet Mites (Trombidiidae) and not the so called 'Red Spiders", also mites, that live and feed on plants and can become pests.
Velvet Mites (Trombidiidae) may be parasitic on insects as young instars. As adults they will become predators of insects. This is the info in BugGuide, Wikipedia and other web locations.

But I am now looking at John Henry Comstock's 1948 Spider Book, revised in 1965 by W.J. Gertsch. I know that is ancient for a science book, but while a good part of the older taxonomy had to be  revised according to more recent phylogenetic research, the life history of the described organisms has not changed and the old biologists were famously thorough observers.

This last paragraph sounds suspiciously like chiggers, doesn't it?

I moved on to the 1998 Review: Biology and Ecology of trombiid mites (Acari: Trombidioidea) by Zhi-Qiang Zhang, Experimental and Applied Acarology, 22 (1998). The paper first states that the systematics of the group are in flux, and many of the old subfamilies that Comstock still included have since been accorded family status.

Euryopis sp. Spider with a trombidioid larvae (red) and trombidioid adult on the left

 These mites are of economic importance and may be used for biological pest control. But identification even to the genus can be difficult and in many cases either the larval stages and hosts are not known or the larvae are described but not the adults. Trombidiid larvae have been found as parasites on many different arthropds. Some seem very host specific, others can use a variaty of hosts.

If you can read this table that I copied out of the Zhang paper, you'll find that two of the families do use vertebrates as their larval hosts: Leeuvenhokiidae and Trombiculidae They are Chiggers!
So the chiggers that chew into the vertebrate skin (not burrow under the skin) are larvae of Velvet Mites, but not all Velvet Mites are chiggers. Here is a link to the blog of Dragonfly Woman where she explains how the young chiggers actually feed on vertebrate skin.
The mites I was asked about are probably not chiggers, for two reasons: First, It is far too early and cold for chiggers on Mt Lemmon right now. We still had snow a couple of weeks ago.  Second, chiggers (the parasitic larvae) are very tiny, much smaller than the observed mites. But I cannot tell whether these mites could be the adults of what we call chiggers.

 Classification:
Arthropoda (Arthropods) » Arachnida (Arachnids) » Acari (Mites and Ticks) » Acariformes » Trombidiformes » Prostigmata » Anystina » Parasitengona » Trombidiina » Trombidioidea » Trombidiidae (Velvet Mites)