Labial palpus

Maxillary palpus

under eyes later in this chapter). For now we retain the more traditional number of head segments pending further investigation.


Compound eyes are present throughout arthropods, although they are absent in Recent Arachnida, Protura, and Diplura, as well as some other more minor lineages, like Sym-phyla. Compound eyes were, however, present in some fossil Arachnida (e.g., Paleozoic scorpions, Trigonotarbida) and trilobites and may have even been present in Paleozoic, xenusian Onychophora (Dzik, 2003). The basic unit of an insect compound eye is the ommatidium. Eyes range from a single ommatidium up to 28,000 ommatidia. For example, a collembolan has two ommatidia, while a dragonfly can have around 28,000. Each ommatidium is composed of multiple cells and divided into two functional units: the dioptric apparatus and the receptor apparatus. The dioptric apparatus is functionally the lens and gathers and focuses light on the receptor apparatus. The receptor apparatus receives the focused light from the dioptric apparatus and translates it into receptor potentials that are sent via neural axons to the optic lobe. In addition to the compound eyes, there are three ocelli (simple eyes) present in most adult insects as well as larvae of Holometabola. Ocelli are composed of a transparent cuticular cornea and cannot form images but are highly sensitive to low light intensity. Simple eyes in holometabolous larvae (e.g., caterpillars) are called stemmata and are structurally similar to ocelli, although they are structurally identical to ommatidia in some mecopteran larvae.

Some researchers believe the eyes are highly modified appendages and thereby evidence for yet another ancestral segment in the head. This theory has some developmental evidence (e.g., Schmidt-Ott et al., 1994, 1995; Scholtz, 1995; Rogers and Kaufman, 1996; Queinnec, 2001) and supports the original hypothesis of this segment (the "ocular segment") by the Russian paleontologist A. Sharov (1966).

Thus, under this theory, the various segments of the insectan head, from anterior to posterior, would be labral, perhaps ocular (perhaps present across all arthropods but reduced or lost several times independently), antennal, intercalary, mandibular, maxillary, and labial.

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