Winter Crane Flies
Most Common species in this area: Trichocera annulata
These insects are common during the cooler winter months throughout the Bay Area and may be found in large numbers from November to May. Adults form outdoor swarms, consisting mostly of males, over bushes or high places and may follow a person around the yard. They are readily attracted to lights and are attracted indoors by light. The larvae of these insects live in decaying organic matter such as leaves, grass, manure, vegetables etc. Warm sunny days during the winter increase their activity and numbers. Cold weather will suppress their numbers and drying of the soil appears to end their occurrence.
These insects do not bite and are primarily a nuisance when they are found in large numbers. They are sometimes mistaken for mosquitoes because of their appearance and flight patterns. Light will attract them, so leaving a porch light off will minimize the numbers that enter your home. Spraying for these insects ususally has little effect since adults will fly in from other areas and are so common during this time of the year.