72 Mî'. W. S. Macleay on the Structure of the Tarsus 



far as I have been able to observe, is a very accurate description 

 of all those sand insects Avhich are allied to Pimelia and Tenebrio. 

 The manner, however, in which a Helops is heteromerous may 

 perhaps be explained by examining the posterior pentamerous 

 tarsus of Erotylus ; in which case we should say that it is the 

 penultimate joint of the tarsus that is evanescent in the Helo- 

 pidw. In the heteromerous Cisfelence of M. Latreille we have a 

 genus Mi/ctcrus, evidently allied to the Curcu/ionichc ; and a very 

 careful dissection has made it appear to me that it is in reality 

 pentamerous, only the third and fourth joints of the posterior tarsi 

 are nearly confluent. Hence, in heteromerous insects generally, 

 Ave may perhaps suppose that it is the fourth joint of the tarsus 

 which vanishes. I say generally, because there may be many ex- 

 ceptions to the rule ; perhaps, for instance, Meloc and the insects 

 allied to it. These are true Heteromcra ; but on looking at their 

 posterior feet we find an obconical process, which seems to re- 

 present the lost articulation, and occasions one almost to fancy, 

 although not perhaps very philosophically, that it is the second 

 joint of the tarsus which articulates with the tibia. The Linnean 

 genus Cassida and Alar nus appear to have only four joints to their 

 tarsus, which dift'ers moreover from that of the Chri/somelidœ, in- 

 asmuch as all these four articulations are dilated. The affinity of 

 Cassida to Chrijsomela shows us here also which joint is evanes- 

 cent ; and we find, in fact, that the last joint, although dilated at 

 the extremity, puts on the appearance of the piece formed by the 

 two last articulations of the tarsus in Chrysomela. This circum- 

 stance may serve to throw light on the structure of the foot in 

 Eumorphus, and the other insects commonly but erroneously 

 called trimerous. I have shown them to be at least tetramerous ; 

 and I conceive, from analogy, that it is the penultimate joint 

 that is here also evanescent. But however this may be, enough 

 has been said to prove the worthlessness of the tarsal system, 



even 



