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The Exoskeleton

In many insects, certain epidermal cells are specialized as exocrine glands.  These large, secretory cells produce compounds (e.g. pheromones, repellants, etc.) that are released on the surface of the exoskeleton through microscopic ducts.

Tiny hair-like projections or surface sculpturing of the cuticle are known as microtrichae or pile (PILL-EE).  These acellular structures consist of a solid core of exocuticle covered by a thin layer of epicuticle.  Larger hairs, bristles, and scales (called setae or macrotrichae) are the product of two specialized epidermal cells:  a trichogen cell (the hair shaft) and a tormogen cell (the socket).  Multicellular projections of the exoskeleton are called spines (or spurs, if movable).  They are lined with epidermis and contain both procuticle and epicuticle.

Skeletal muscles attach to the inner surface of the integument.  Despite small body size, insects have many more muscles than vertebrates because the exoskeleton affords a larger surface area than an endoskeleton (relative to body volume) for muscle attachment.  An insect owes its incredible strength to the geometry of its musculature -- providing optimal leverage for movement of appendages.

Invaginations (inward folds) of the exoskeleton add to its strength and rigidity.  They also provide increased surface area for attachment of muscles.  Ridge-like invaginations are called apodemes.  They are usually visible externally as a groove (suture).  Finger-like invaginations are called apophyses.  A tiny pit usually marks their location externally.

The colors found in the integument of insects are produced either by pigment molecules, usually located in the cuticle, or by physical characteristics of the integument that cause scattering, interference, or diffraction of light.  Pigments that are frequently present include the pterines, melanins, carotenoids, and mesobiliverdin.

Color patterns may change over time.  Rapid, temporary changes may occur in response to daily environmental conditions or to the threat of danger.  Slower, more permanent changes are usually related to seasonal changes in the environment or hormonal influences.

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