EXTERNAL ANATOMY OF J N SECTS. 683 



Pentamerous insects are those which have five joints in 

 all their tarsi. This is the most universal, and may be 

 called the natural number of these joints. More than half 

 the Coleoptera belong to this section ; in the Orthoptera 

 — the BlatticUc, Mantidce, and Phasmidce : all the Lepi- 

 dopiera except those butterflies called tetrapi [Vanessa, 

 &c); all the Trickoptera, Uymenoptera, and Diptera : 

 in the Neuroptera — Ascalaphus, Myrmeleon, Hemerobius, 

 Corydalis, &c. ; and in the Aptera — Pulex a . 



Heteromerous insects are those in which the number 

 of these joints varies in the different pairs of legs b . These 

 variations, like the spurs, may be expressed by three 

 figures, the first representing the anterior tarsus, the 

 second the intermediate, and the third the posterior. I 

 begin with 5:5:4!. This number represents those beetles 

 that have been exclusively regarded as heteromerous by 

 modern Entomologists — of this description is the Lin- 

 nean Tenebrio, Meloe, &c, now subdivided into nume- 

 rous genera ; they have Jive joints in the two anterior 

 pair, and four in the posterior. The tarsal joints of the 

 aquatic genus Hydroporus (a singular anomaly in the 

 Order to which they belong) are expressed by 4:4:5, 



a The CleridcE, which M. Latreille has placed in the pentamerous 

 section, vary considerably in the number of their tarsal joints. Thus 

 in general in Thanasimus the tarsi are pentamerous ; but in T.for- 

 micarius they appear to be heteromerous ; and in Enoplium, Opilo, 

 Clerus and Necrobia they are ietramerous. M. Latreille's expression, 

 (2V. Diet, a" Hist. Nat. vii. 172.) " le premier article etant fort court et 

 cache sous le second," seems to indicate that there is a fifth joint in 

 some of these, the first being concealed under the second ; but I have 

 never been able to discover it. Perhaps he reckoned the pulvillus as 

 a joint ? 



b The term heteromerous properly belongs to all insects in which 

 the different pairs of tarsi vary inter se in the number of their joints, 

 and it is here used in that lanje sense. 



