398 SYSTEM OF INSECTS. 
the Anoplura and Thysanura; the Chilopoda and Chi- 
lognatha amongst Apterous insects; in the Scorpionidea 
and Aranidea amongst the Arachnida; and in the Ma- 
crura and Brachyura amongst the Decapod Crustacea. 
Again, in other cases three seems to be the most promi- 
nent number: this takes place sometimes with regard to 
the primary groups of an Order, or what I denominate 
the Suborders. Thus we have the Diurnal, Crepuscular, 
and Nocturnal Lepidoptera; the Linnean genera Blatta, 
Mantis, and Gryllus constitute the Orthoptera ; and other 
instances of this number might be produced in some 
minor groups. But that which appears to prevail most 
widely in nature is what may be called the guaterno-qui- 
nary; according to which, groups consist of four minor 
ones; one of which is excessively capacious in comparison 
of the other three, and is always divisible into two; which 
gives five of the same degree, but of which, two have a 
greater affinity to each other than they have to the other 
three*. Mr. W.S. MacLeay, in the progress of his in- 
quiries to ascertain the station of Scarabeus sacer L., dis- 
covered that the thalerophagous and saprophagous Pe- 
talocerous beetles resolved themselves each into a circle 
containing five such groups. And having got this princi- 
ple, and finding that this number and its multiples pre- 
vailed much in nature, he next applied it to the Animal 
Kingdom in general: and from the result of this investi- 
gation, it appeared to him that it was nearly, if not alto- 
gether, universal”. Nearly at the same time a discovery 
almost parallel was made and recorded by three eminent 
* Linn. Trans. xiv. 56—. It is to be observed, however, that 
what Mr. MacLeay calls the aberrant groups are usually also resolv- 
able into two. » Hor, Entomolog, 318, et passim. 
