November, 1912. 



KNOWLEDGE. 



435 



MICROSCOPY. 



By F.R.M.S. 



THE HARVEST MITE.— I was very pleased to see Mr. 

 Marriage's photomicrograph of the Harvest Mite in the 

 August number of " Knowledge," page 317, because it 

 entirely agrees with some specimens sent to me by Dr. 

 George, of Kirton Lindsey, in 190S. These specimens were 

 sent to me alive and I made drawings of one. dorsal and 

 ventral surface (see Figures 467 

 and 468). which w.as printed with 

 Dr. George's article in The 

 Naturalist for September of 

 that year. What makes it still 

 more interesting is that none of 

 the previously-published figures 

 I have seen agree so well 

 with Mr. Marriage's photo- 



FlGURE 460. 



micrograph as my own. This 

 may be accounted for by there 

 being more than one larval 

 form of the Acarina on the war 

 path at the same time, which 

 is always during the first active 

 part of their existence, the 

 larval stage immediately after 

 leaving the egg. The larval 

 forms of the Hydrachnids are 

 always parasitic at this stage, but not afterwards. 



Of course, there is no doubt about it being a larval 

 form of some mite or other ; but which mite is the 

 question we have yet to settle. The larvae of 

 Troinbidiuin holosericeutn and Trombidium 

 fiiligiiiosuin are both well known. I have a number 

 of each of them myself, but they do not at all agree 

 with the one on page 317. Neither do we know 

 of any one being bitten by either of them. .A 

 naturalist in Edinburgh bred some T. Iiolosericeiini 

 larvae in confinement and placed the living larvae in 

 his stocking but did not succeed in getting bitten. I 

 tried the same experiment this year with T. fuUginosnin 

 with the same result — no bites. We also know by- 

 description and figures a number of larval forms of Trom- 

 bidiums ; but none agree with the larval form which we are 

 sure causes the mischief that to some people is so distressing. 

 It makes one wonder if the irritating little creature is a 

 Trombidium at all. If some one could only succeed in keeping 

 some alive long enough to reach the next stage it might help 

 us very considerably in determining its proper place in the order 

 .■\carina. But at present we are quite in the dark as to its 

 proper name, except we know it as Trombidium aittiimnalis. 



The superfamily Trombidoidae is divided into six families — 

 Caeciliidae, Cheyletidae, Erythraeidae, Tetranychidae, Rhyn- 



FlGURE 461. 



Figure 464 



Figure 466. 



to give them their proper names. But when we do know, it is 

 no doubt amongst one of the above six families we shall find 

 them. 



In the Autumn time the Harvest Mite is sure to turn up in 

 person and in print. Some newspaper or other is sure to 

 bring it forward, either seriously or humorously ; even Punch 

 had a joke out of them in 1907. But, however much has 

 been printed, none have yet helped us to form a correct 

 diagnosis of its true position in the .\carina. The bibliography 

 of the Harvest Mite is very extensive, and covers a wide range 

 of literature. Let us just glance at some of the references 

 which are of easy access to the amateur naturalist. 



The earliest account I have found is Baker's " Employment 

 of the Microscope " (1753), page 343, Figure 1. Baker says: — 

 " I have sometimes suspected this little creature might be a 

 young sheep-tick from its figure and way of burying itself, but 

 then it should be rather found where sheep feed, than in corn, 

 and before sheep are suffered to come into those fields, and 

 it is never got as I have heard in grass fields, unless bordering 

 upon corn, but amongst wheat it never fails. If any one has 

 a mind to make trial upon this insect, how it comes to be 

 amongst corn only and yet live by sucking of blood, he may 

 easily find abundance of them ; for though they prefer the 

 ladies, yet they are so voracious, that they will certainly lay 

 hold of any man's legs that come in their way." Here is the 

 opinion of Baker : he thinks it is a young sheep-tick, and his 

 figures certainly help the conclusion. (Figure 460.) 



('.ilbrrt White in Letter XLIII mentions it, but does not 

 attempt tocallit anythingbut acarus. 

 Kirby and Spence also mention 

 it. but help us no further with the 

 name than Lcptus autitmiialis. 



Knchenmeister gives an account 

 of this mite and a figure by 



cholophidae and Trombididae, and these again into a large 

 number of genera. The adult forms of the Harvest Mites are 

 probably quite well known ; it is only our ignorance as to 

 which adult to ascribe the larval forms which does not allow us 



Figure 462. 



Professor Leuckart. This 

 figure is the nearest to Mr. 

 Marriage's photomicrograph 

 I have seen, and I should 

 think is intended to re- 

 present the same creature, 

 but he calls it by the same 

 name as Kirby and Spence. 

 (Figure 461.) 



Banks, the American, 

 writing on mites, says it is 

 the larval form of a 

 Trombidium. He does not 

 mention any particular 

 species but he gives a figure 

 of the larvae of a Trom- 

 bidium, which is quite different from the one in question. 

 (Figure 462). 



Murray, in " Economic Entomology," calls it Tetranychus 

 autiimnalis. gives a figure of the same, and takes it out of 

 the family Trombididae ; but the figure Murray gives is 

 very different from Mr. Marriage's photograph. (Figure 464.) 



The " Micrographic Dictionary," calls it Trombidium 

 atttiimnalis. and gives a figure with eight legs, but says the 

 young form with six legs is frequently met with. (Figure 463.) 



■'The Cambridge Natural History," Vol. IV., dismisses the 

 the subject in the following few words : — " The Trombididae 

 include most of the moderate-sized velvety red mites which 

 are commonly known as Harvest-mites, and their larvae, the 

 so-called Harvest-bugs, frequently attack man. Trombidium 

 liolosericctim is a well-known example." No Figure is given. 



Figure 463. 



