Hello Gyro,

The movement of materials into the gut and hemolymph is dynamic.  Remember
that the foregut and hindgut are sclerotized, so absorption of food
nutrients can only occur in the midgut.  Food is ingested through the
proboscis and mouth, and digestive enzymes can be added from the salivary
and hypopharyngeal glands, which are in the head.  Food can be either
pollen or nectar/honey or jelly.  If chemicals are entering with
contaminated pollen, the filtering mechanism of the proventriculus may
affect movement into the midgut.  In the midgut, nutrients are absorbed and
transported across the midgut epithelium into the hemolymph.  Many basic
food molecule are then stored as macromolecules in the fat body -- which is
primarily in the abdomen of workers.  Sugar monomers are often stored as
glycogen, and the triglycerides become fats.  Hormones regulate the
introversion of food molecules from the storage forms into the
free-hemoymph forms.  The Malpighan tubules play a crucial role in
eliminating metabolic wastes and chemicals from the hemolymph -- a role
analogous to the human kidney.  Portions of the hindgut are involved in
water recovery back into the hemolymph.  So, the fate of any chemical may
be difficult to predict.  Most insecticides are lipophilic -- so they are
readily absorbed by the wax layers of the cuticle, and cuticular barriers
can slow movement of chemicals into or out of the insect body.
 Additionally, various enzymes attack xenobiotics and break them down into
water soluble compounds that can be eliminated in the urine.  These enzymes
include cytochrome mono-oxygenases, glutathione-S-transferases, etc.  Often
the water soluble metabolites get concentrated into the urine.  So,
chemicals can be concentrated in the abdomen during the early stages of
digestive absorption, and they can also be traced in the forms of breakdown
products in the abdoment as the Malpighan tubules concentrate them.  There
may not be a general pattern that can explain tissue concentrations for all
chemicals or even chemicals within a particular class -- rates of all of
these processes vary with the substrate chemical structure.

Jeff Harris

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