Sucking lice and spiracular transpiration: turning a liability into a benefit and a necessity

Sucking lice feed on blood and therefore ingest more water than they require for maintaining hydration. This water must be excreted; however, unlike other blood-feeding insects, they do not produce urine but do become dehydrated within hours if unable to feed. Using human clothing lice and head lice, Pediculus humanus ssp., and high sensitivity balances, it was shown that recently fed lice lost mass consistently as water ingested with the blood meal was excreted via the respiratory system. If all spiracles were occluded using petroleum jelly, mass/water loss was inhibited. Blocking thoracic spiracles resulted in a slight reduction in the rate of mass loss compared with untreated lice, but blocking the abdominal spiracles resulted in an enhanced rate of mass loss. Lice immersed in water did not lose mass but maintained the same mass for several hours, after which they increased in mass as the tissues became turgid, indicating that the insects were able to block water ingress during the period of stability, but that after some time the mechanism failed allowing water to enter the lice by osmosis.