202 



THE AGRICULTURAL NEWS. 



June 25, 1910. 



INSECT NOTES. 



THE ACARINA OR MITES. 

 PART I. 



The articles which have appeared in the Agricultural 

 Neics under the title of Natural History of In.sects, and those 

 which gave accounts of the several orders of insects, were 

 intended to provide a brief outline of entomology. Mites and 

 ticks are included by many persons among the insects, and 

 though they are not true insects, they are closely related. It 

 is now intended to give brief accounts of certain mites and 

 ticks, showing how they differ from insects, and giving also 

 the principal groups into which they are divided. 



In order to present a fair idea of the position which the 

 mites and ticks occupy with relation to nearly connected forms, 

 it will be necessary to review the divisions of the Arthropoda, 

 and of the Arachnida - that class of the Arthropoda in which 

 is to be found the order Acarina, to which the mites and ticks 

 belong. 



References to the papers on the Natural History of 

 Insects, and on the orders of insects, will be found in the 

 Agricultural News, Vol. VIII, p. 410 It would be well for 

 readers to refer to tnose articles, since many terms are 

 explained there which will be used again in this and succeeding 

 instalments. 



The order Acarina includes a great many forms of 

 insectiike animals which are of interest to the agriculturist. 

 It is, therefore, proposed to give a brief account of the order 

 in a manner similar to that in which the true insects have 

 been discussed. 



It will be remembered that the arthropods, or jointed- 

 foot animals, are divided into four classes :— 

 Class I. Crustacea — Crabs, Lobsters, etc. 

 Class II. Arachnida — Spiders, Mites, Ticks, Scorpions, etc. 

 Class III. Myriapoda — Centipedes, Millipedes, etc. 

 Class IV. Hexapoda — Insects. 



Tun Arachnida. Theclass Arachnida, which includes the 

 mites and ticks, with other forms, is an extensive group. It 

 is apparently very ancient in its origin, and its present day 

 forms show much diversity in size, general appearance and 

 habits. 



The members of this class agree, as a whole, in certain 

 characteristics, chief of which are the absence of antennae, 

 the absence of compound eyes, and the presence in the adult 

 of four pairs of walking legs, in all of which points they differ 

 from the insects. They are never winged, and this is 

 a further point of difference between them and the true 

 insects. The Arachnida are air-breathing, the more common 

 breathing apparatus being the pulmonary sac or lung-book, in 

 which the respiratory tissue is arranged in plates like the 

 leaves of a book. In certain forms, theve are extended tubes 

 resembling the tracheae of insects, though probably not 

 homologous with them. Certain forms also effect respiration 

 through the surface of the body, withcut any special organ 

 for the jjurpose. 



The mouth parts \ary considerably, but usually consist 

 of strong, biting mandibles. These are sometimes modiffed 

 to form a piercing organ, as in the case of the ticks. 



The body regions are more or less fused, the head and 

 thoracic regions forming a cephalothorax, which bears the 

 head appendages and the walking legs. The abdominal 

 region in the spiders and scor|)ions is distinct, and in the 

 latter is segmented. In the mites, the abdomen is fused wnth 

 the cephalothorax, so that there is no distinct line of separ- 

 ation of the bod}' into .structural regions, though in certain 

 forms, segments or body rings are distinguishable. The 



nervous system is ventral, and similar to that of insects. 



The eyes are simple, or in many parasitic forms they 



may be absent. The circulation, also, is .similar to that of 



insects, including usually a large dor.sal vessel. The sexes 



are distinct. 



The class Arachnida is composed of related orders, a list 



of which is as follows : — 



Scorpionida Scorpions 



Pseudoscorpiouida Book Scorpions 



Pedipalpi Whip Scorpions 



Solpugida 



,,, , ., I'Harvestmen, or 



Ihalangida - T^ 1 1 i i 



° 1^ Daddy-longlegs 



Ariiueida Spiders 



Acarina Elites and Ticks 



Linguatulida 



Tardigrada 



Pycnogonida 

 suoRi'ioNiDA. The scorpions are among the oldest, geo- 

 logicaliy, of all the arthropods. They are at present mostly 

 to be found in tropical and subtropical countries. They 

 have a broad cephalothorax, and an abdomen of thirteen 

 segments, with a sting and poison gland in the last of these. 

 The abdomen may be seen to be composed of two distinct 

 regions, the 'pre-abdomen' of seven segments, and the 'post- 

 abdomen' of six. The second pair of appendages, the pedi- 

 palps, are enlarged and chelate, i.e., provided with a pincers- 

 like structure. The four p.iirs of walking legs are about equal 

 in size. The sting is venomou.s, and though rarely fatal to 

 man, is to be regarded as distinctly dangerous. 



psEUDoscouPiONiDA. The book scorpions are minute 

 arachnids of very little economic importance. They probably 

 feed on sniad insects under bark of decaying wood, among 

 old books, etc., where they are generally found. 



I'KrtiPALPi. The whip scorpions inhabit the tropics and 

 warm regions. They differ from the preceding in the structure 

 of the pedipalps, which are smaller and less strongly chelate. 

 The first pair of legs is elongated to form a whip-la.sh-Iike 

 structure. The common lorm in the West Indies is the tail- 

 less whip scorpion, in which the abdomen is very short and 

 flattened. The bite of the members of this order is sa'd tc be 

 venomous. The more common forms are commonly called 

 tarantulas or .scorpions in some islands. 



I'li.^LANciDA. The harvestmen or daddy-long-legs some- 

 what resemble spiders. They have small, oval, or rounded 

 bodies and very long, slender legs. They differ from the 

 spiders in not having the abdomen attached to the cephalo- 

 thorax by a stalk, and in not possessing any spinning glands. 

 They feed upon flies and other insects, and are more abundant 

 in temperate countries than in the tropics. 



Ai;.ANEi7iA. The spiders are the most conspicuous, and 

 probably the most widely distributed of the Arachnitla. The 

 body is divided into cephalothorax and abdomen, joined 

 together by a slender pedicel. The web-spinning habits of 

 the.se animals render them conspicuous, and their .sagacity 

 in all that pertains to the capture of their prey has attracted 

 much attention to their habits. They are useful in that 

 of capturing inserts, and though certain species are con- 

 sidered venomous^' they arc not often capable of inflicting 

 serious wounds on' man. Itespiration, in the spiders, is by 

 means of lung-books and tracheal tubes. The sjnnning glands 

 are situated near the end of the abdomen. 



ACAP.iN'A. This order includes the red spiders, poultry 

 and bird inites, ticks, cheese mites, itch, mange and scab 

 mites, and the gall' mites. They have' four pairs of walking 

 legs, as adults, but^often only three pairs when young. The 

 month parts are formed for biting or piercing, sometimes for 



