. A text-book of invertebrate morphology. Invertebrates. I'YPE TRAGHEATA. 489. or two unsegmented plates of which the innermost is usually toothed. The second maxillae (Fig. 225, A) are also jointed and are fused together to form a lower lip, or labium. The fused basal joints form the suhmentum {sm), the second joints the mentum (m), which bears, as does the stipes of the maxillae, a jointed palp {p) and terminates frequently in two unjointed plate-like processes. The three pairs of appendages of the thorax are typically ambulatory, but are modified for clasping, swimming, digging, etc., accor
. A text-book of invertebrate morphology. Invertebrates. I'YPE TRAGHEATA. 489. or two unsegmented plates of which the innermost is usually toothed. The second maxillae (Fig. 225, A) are also jointed and are fused together to form a lower lip, or labium. The fused basal joints form the suhmentum {sm), the second joints the mentum (m), which bears, as does the stipes of the maxillae, a jointed palp {p) and terminates frequently in two unjointed plate-like processes. The three pairs of appendages of the thorax are typically ambulatory, but are modified for clasping, swimming, digging, etc., according to the habits of the insect. They typically con- sist of a basal joint, the coxa, succeeded by one or two small joints, the trochanter, upon which follow a femur, a tibia, and a tarsus, the last consisting of five (occasionally four) short joints, the terminal one bear- ing two claws or ungues. The abdomen in the adult forms is as a rule, destitute of appendages, except in the Thysanurans (Fig. 231), the lowest of all the orders of Insects. In these a number of the segments are provided with a pair of spurlike processes which recall the spurs upon the basal joints of the trunk appendages of Scolopendrella, and are apparently ho- mologous with them. In the embryos of probably all forms rudimentary appendages are found on some of the abdominal segments, but they later disappear, showing, however, a de- scent of the Insecta from forms in which abdominal appen- dages were functional in the adult. Processes of various kinds, such as cerci, ovipositors, and copulatory organs, are frequently borne by the posterior abdominal segments, but these do not seem to be equivalent to appendages. As stated, a pair of wings is usually borne by the meso- and metathoracic segments. These structures are entirely wanting in the lowest insects, the Thysanura and CoUembola, Fig. 235.—Mouth-parts of a Bee- TLB, Monohammus. A = labium. B = maxilla. G = Please note that these images are
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