62 
TRANSACTIONS OF THE TEXAS ACADEMY OP SCIENCE. 
of Mexico; and throughout the tropics where there is green vegetation, 
especially in the lowlands or tierra caliente. Their comparative absence 
during any part of the year in this region is solely dependent on the oc- 
currence of frost or drouth. In the bottom of the Grand Canon of the 
Colorado, north of Flagstaff, Arizona, from 4000 or 5000 feet down to 
2500 feet (the level of the river), they were quite bad early in July; 
indicating the locality as bordering on the Lower Sonoran. In the Mes- 
illa Valley, they are troublesome sometimes to a certain extent, in June 
and J uly, but are less numerous than I found them in the Grand Canon, 
and are not a circumstance to what they are at Washington. As regards 
abundance, allowances must be made for the comparative differences in 
the extent and rankness of green vegetation in the arid and humid re- 
gions. 
These larval forms of Trombidium have never before been referred 
to in connection with the subject of biogeography, and I am glad to be 
able to find some use for them (as I have never had any use for them be- 
fore!). I can testify that they form an excellent criterion, if taken in 
June or July, in green and rankly-growing vegetation, grass, or thickets, 
but not during a time of drouth. These creatures are very sensitive to 
a high temperature combined with great dryness, but probably only be- 
cause these conditions destroy the sueculency of the vegetation upon 
which they are vitally dependent. Even in the tropical lowlands of Mex- 
ico, I have known them to quite disappear at times from April to June, 
in consequence of extremely dry and hot weather, this being the usual 
height of the dry season in the lowlands of the coast. I have also ob- 
served them to disappear in southern Texas, on the lower Eio Grande, 
in August, after a month or two of very hot dry weather. Such weather 
greatly withers the grass and all the vegetation, upon the juices of which 
these larval mites feed. They never enter the green and grassy Transi- 
tion of the mountains of New Mexico and Arizona; and hence these re- 
gions form a perfect haven of rest to the weary field naturalist, who has 
done battle for months with the coloradillas, pinolillos and garrapatas 
(adult ticks) of the tierra caliente. They are present in the Upper Sono- 
ran of New Mexico and Arizona only in the low valleys of streams, where 
there is much green vegetation. I have never noticed them in the val- 
leys of the higher parts of the Upper Sonoran , though they may occur 
there sparingly. 
On the Partial Extension of the Lower Sonoran Fauna up 
River Valleys into the Upper Sonoran Province. — It is well 
known that some Lower Sonoran forms extend at times up river valleys 
for a long distance into the Upper Sonoran. The presence of such forms 
in the Rio Grande Valley guided Dr. Merriam in extending his Lower 
