6 The Structure and Special Physiology of Insects 
the segments of the abdomen retain their independence and are more or 
prothorax^ 
labiaMk 
■palpi '^ 
proboscis' 
\ \ metathorax 
\\ mesothorax 
N s ''.coxa 
''trochanter 
'''"femur 
'abdomen 
compound eye, 
antennse. 
tarsal segments 
Fig. 7.—Body of the monarch butterfly, Anosia plexippus , with scales removed to show 
external parts. (Much enlarged.) 
less similar, thus preserving a generalized or ancestral condition. On the 
head are usually four pairs of jointed appendages (Fig. 8), viz., the 
antennae and three pairs of mouth-parts, 
known as mandibles, maxillae, and labium or 
under-lip. Of these the mandibles in most 
cases are only one-segmented, while the two 
members of the labial pair have fused along 
their inner edges to form the single lip-like 
labium. The so-called upper lip or lab rum, 
closing the mouth above, is simply a fold of 
the skin, and is not homologous, as a true 
appendage or pair of appendages, with the 
other mouth-parts. In some insects with highly 
modified mouth structure certain of the parts 
may be wholly lost, as is true of the mandibles 
of dobson-fly, Corydalis cor- in the case of all the butterflies. The head 
nuta, female, showing mouth- bears also the large compound eyes and the 
parts, lb., labrum, removed; . 0 ... 
md., mandible; mx., maxilla; smaller simple eyes or ocelli (for an account of 
li., labium; gl, glossse of la- the eyes see p. 30). Attached to the thorax are 
mxf., palpus of maxilla; ant., three P alrs of kgs, whlch are jointed appendages, 
. antenna. homologous in origin and fundamental struc¬ 
ture with the mouth-parts and antennae, and two pairs of wings (one or 
Fig. 8.—Dorsal asDect of head 
