Box 2. The insect gut
The insect gut is divided into three regions; foregut, midgut and
hindgut. Of these the first two are continuations of the ‘outside’ of
the insect and are chitin-lined, so that their surfaces do not present
areas of exposed cells (although receptors and transporters are
present to allow processes such as taste recognition in the mouth
cavity and water transfer in the hindgut to occur). The midgut region
is the only part of the gut that contains surfaces of exposed cells,
and it is the main site of exchange between the circulatory system
(haemolymph) and the gut contents. The midgut itself is responsible
for nutrient absorption, whereas excretion and water balance take
place primarily in the Malpighian tubules attached to the hind end,
which carry out a function similar to that of the kidney in higher
animals. RNAi effects occurring in insects as a result of oral delivery
of dsRNA are presumably mediated by the midgut surfaces through
exposure of cells of the midgut epithelium and the Malpighian
tubules to dsRNA in the gut contents.
Conditions in the gut vary considerably between insect orders.
Gut pH is an important factor in insect digestion and can vary from
predominantly acidic (coleopteran larvae) to strongly alkaline (up to
pH 10.5 in some species of Lepidoptera). In addition, within a single
insect the pH changes along the gut and with distance from the gut
epithelium. The stability of ingested dsRNA in the insect gut could
be affected both by chemical hydrolysis (which increases with
increasing pH) and by enzymes present in the gut contents.