EXTERNAL ANATOMY OF INSECTS. 685 



lates in the summit of the ball, and is moved by appro- 

 priate muscles 3 . This structure probably permits the 

 readier elevation and depression of this joint. In the 

 Orthoptera the tetramerous genera are those which 

 Linne called Tettigonia amongst his Grylli {JLocusta F.); 

 Acheta monstrosa also, and in the Neuroptera, Raphidia 

 belong to this section. 



Trimerous insects are those whose tarsi consist of only 

 three joints. Amongst beetles the Lady-birds (Cocci- 

 nella L.) are remarkable for this structure, but in them 

 the claw-joint is also Particulate, so that strictly speak- 

 ing they are tetramerous ; in the Orthopterous Order the 

 migratory locusts (Locusta Leach) belong to this sec- 

 tion, as likewise Gryllus Latr. and Gryllotalpa Latr. : 

 in the first of these genera is an appearance of there be- 

 ing more joints in the tarsus, because there is more than 

 one cushion below the first b . To this section also belong 

 the great majority of the Hemiptera, excluding only those 

 tribes that connect the two sections of the Order consti- 

 tuting the two Linnean genera Nepa and Notonecta ; the 

 Libellulina likewise belong here, as do also the Scor- 

 pionidae and Scolopendrida. 



Dimerous insects are those that have two joints in 

 all their tarsi. Such are the Pselaphidte in the Co- 

 leoptera Order ; in the Hemiptera — Belostoma and No- 

 tonecta; in the hexapod Aptera — Pediculus; in the octo- 

 pod— the Acari of Linne; in the myriapod — Iidus; and 

 in the Arachnida — the Araneidce. 



a Plate XXVI. Fig. 49. s: a. b Vol. II. p. 330. 



a Dr. Leach says there are three joints in this tribe. Nat. Misc. 

 iii. 80. 



