The mechanism by which arboviruses bypass the basal lamina of mosquito midgut cells and enter the body cavity has been unclear. Experiments using Venezuelan equine encephalitis viral replicon particles, which express the green fluorescent protein gene in cells, indicate the operation of tissue conduits, possibly involving tracheae and visceral muscles, that facilitate virus movement through the basal lamina. Ultrastructural studies of the midgut reveal evidence for possible complete penetration of the basal lamina by tracheal cells and regions of modified basal lamina associated with visceral muscle. The modified basal lamina closely resembles proventricular matrix material known to allow virus passage.