|Taken from bugguide.net|
Mantisflies (Mantispa sayi) are pretty weird looking creatures. Imagine somebody shrinking a praying mantis and attaching its front end to the hind end of a lacewing. Don’t know what a lacewing is? No problem, just check out this Wikipedia page.
The adult mantisfly is a predator, using its big front legs to catch small insects in the same manner as a praying mantis. As larva they are predators of spider egg sacs.
A larva hatches from its egg and may seek a spider upon which it rides until its host spins an egg sac. If the spider happens to be a male, the larva will wait until it mates, in which case it moves over to the female. Once inside an egg sac, the larva changes into a grub that feeds on the eggs and hatched spiders until nothing is left. Then it pupates and later an adult emerges from the empty egg sac. The adults can have very different sizes depending on the amount of food they had as larva. This is very unusual in the insect world as most other species have more or less similar sized adults.