16 
POPULAR SCIENCE REVIEW. 
highest expression in the complex brain of man and the Verte- 
brate animals. 
The eight principal orders of six-footed insects have been 
arranged by Packard into the two following groups of Metahola 
and Heterometabola,* viz. 
Sub-orders, Packard. 
Metabola. The body usually cylindrical ; pro thorax ^ 
small ; mouth parts more generally haustellate Hymenoptera. 
(formed for sucking) ; metamorphosis complete ; Lepidoptera. 
pupa inactive ; larva usually cylindrical, very Diptera. 
unlike the adult. J 
Heterometabola ; the body usually flattened ; pro-^ Coleoptera. 
thorax large and squarish ; mouth parts usually | Hemiptera. 
adapted for biting ; metamorphosis in a large bOrthoptera. 
number incomplete ; pupa often inactive ; larva I Neuroptera. 
flattened, often resembling the adult. J Thysanura. 
In the Metabola, as the higher of these two groups, the 
differentiation of the thorax from the other parts of the body 
is more marked than in the other. The three segments of 
which it is composed, and which are known as the pro-, meso-, 
and meta-thorax, have coalesced together into a compact mass, 
separated by deep incisions from the head and abdomen.* * * § )* The 
function of flight is chiefly concentrated in the mesothorax,* 
which segment is enormously enlarged at the expense of the 
other two. In the Heterometabola, on the contrary, the 
evidences of imperfect specialization are seen in the greater 
separation of the thoracic segments from each other, and their 
closer connexion with the abdomen. § The power of flight is not 
so much confined to one segment, viz. the mesothorax, but is 
sometimes, as in the Coleoptera, resident in the meta thorax, and 
sometimes, as in the Orthoptera and Neuroptera, the property of 
both ; or, again, it may be absent altogether, as in the Thy- 
sanura. || In conformity with this, the nervous system in 
the Metabola, as also in the Coleoptera, as the highest order of 
Heterometabola, exhibits a greater amount of concentration than 
* It should be observed that these two groups do not form a linear 
series, but rather two parallel ones. See Packard’s Guide to the Study of 
Insects , p. 105. 
t The regional division of the body is more marked than its division into 
segments. 
t See figs. 6 and 7 in illustration. 
§ In other words, segmental still prevail over regional distinctions. 
|| In the Hymenoptera and Lepidoptera there are two pairs of wings, 
but in the former the hind pair are not provided with muscles of flight, and 
in the Lepidoptera they are imperfectly so, the important longitudinal 
muscles being absent. I therefore regard them as subordinate to the front 
pair, and have described the function of flight as chiefly concentrated in the 
mesothorax. 
