I hypothesized that for saproxylic insects, a habitat is fragmented when spatial connectivity of dead wood pieces (coarse woody debris) is low. In a two-year study, I investigated the influence of volume and connectivity of pieces of dead wood on species richness and species composition of saproxylic Diptera and Coleoptera in a mixed beech-spruce forest reserve in Switzerland. Saproxylic insects are dependent during some part of their life cycle on dead wood, wood-inhabiting fungi, or on the presence of other saproxylic species. Volume and spatial distribution of dead wood pieces were recorded on scales ranging from 50–200 m around the plots where the insects were collected. Relationships between dead wood connectivity and species richness existed only on the 150 m scale. Plots with high dead wood connectivity had more species than plots with clumped distributions of dead wood pieces. Different species of saproxylic Diptera and Coleoptera were caught in plots with high versus low dead wood connectivity. Hence, for saproxylic insects, habitat fragmentation occurs at a local scale through the isolation of single dead wood pieces. Dead wood spatial distribution is therefore crucial for the survival of these insects and should be considered in forest management plans.