More everyday insects from our lives Fly Head Flies make up the fourth largest insect order. There are about 119,500 known species of flies, and probably a lot more not yet described.
Most flies are active during the day and so have large compound eyes on the sides of their heads.
Many of them also have smaller simple eyes on the top of their heads to detect light levels. This one has four little ones on its head, colored green here. The long red hairs on the antennae, head and body are mostly sensory setae. This material may not be reproduced in any form without express written permission.