Description & Statistics

Hemerobiidae are known as "brown lacewings" and are distributed worldwide. There are around 60 species known in North America. Diagnostic characters include forked costal cross-veins, filiform or moniliform antennae and the forewing having two or more R-s veins.

Hemerobiids resemble chrysopids as far as their predaceous habits are concerned, but they may be distinguished on the basis of several subtle characteristics as follows: Hemerobiid larvae never carry trash as do some chrysopid larvae; the eggs are not stalked and are attached by their sides to leaves and bark (Withycombe 1922, 1923). Eggs are whitish, grayish or even pink and elongate-oval, with the chorion often pitted. They bear a knob-like or disk-like micropyle, often of considerable size (Clausen 1940/62). The chorion is densely studded with glossy papillate projections. The egg breaker is saw-shaped. Eggs of H. pini are pale cream colored, that changes to brown before hatching (Clausen 1940/62).

There are three active larval instars, all quite similar to the larvae of Chrysopidae. Only 1st instar larvae have the trumpet-shaped tarsal claws, while they occur on all instars of Chrysopidae. No species carries a trash packet on the dorsum of the body and thus none has the hooked setae.

The cocoon is loosely woven, double-walled, elliptical in outline, and generally is formed in protected spots, such as under bark, rather than exposed on bark or foliage. Cocoons differ markedly from the nearly spherical parchment-like ones of Chrysopidae. The pupal skin is cast outside the cocoon.

All species are predaceous, attacking mainly aphids, but also chermids, mealybugs and white flies, and sometimes diaspine scales. Several species are restricted to certain types of trees and thus feed only on insects that live on those trees.

Withycombe (1922, 1923) gave early accounts on the behavior of a number of Hemerobius spp.. Hemerobius pygmaeus Ramb. is confined entirely to oak, while H. atrifrons McL. and H. pini Steph. are found only on conifers (Killington 1932). Killington (1936, 1937) gave detailed information on immatures and behavior of several species.

There are several generations annually, the number varying with climate. Sympherobius amicus has 8 generations in Israel, with the minimum cycle taking only 23.5 days. Hemerobius pygmaeus and several others have only a single generation in England. The average duration of the egg, larval and cocoon stages in summer in temperate regions is ca. 5-12, 10-18 and 12-16 days, respectively. Most species hibernate as either larvae or pupae within the cocoons, though H. stigma Steph. can be found as adults during winter and H. pygmaeus occurs only as free larvae in sheltered places. Wesmaelius concinnus Steph. hibernates in the egg in England, this stage persisting for 7 months.

There are two color forms of adults in many species, the normal summer form being brown, while that found during winter is grayish-brown or spotted (Withycombe (1922, 1923).

Bodenheimer (1928) recorded an average production of 250 eggs by a series of females and a maximum of 589 by one individual. Smith (1923) studied the behavior of a series of North American species and found that the gestation period took ca. 5 days. A maximum production of 460 eggs was obtained from Hemerobius humuli L., though this was from a field-collected female and may not represent the total capacity. Micromus posticus Wlk. lays up to 58 eggs per day. Clausen (1940) regarded the reproductive capacity to be somewhat higher than in Chrysopidae.

Several species have been imported for biological control of forest pests in Canada and other countries. Bodenheimer (1928) claimed effective control of Pseucococcus citri Risso in Israel through the liberation of large numbers of native Sympherobius amicus Nav. Several species of Hemerobius were introduced into Australia, New Zealand and Canada for control of Chermidae attacking forest trees.

References: Please refer to <biology.ref.htm>, [Additional references may be found at: MELVYL Library]

Banks, N. 1906. A revision of the nearctic Hemerobiidae. Trans. Ent. Soc. Amer. 32: 21-51.

Killington, J. F. 1932. Notes on the life-history of Hemerobius pini Steph. Trans. Ent. Soc. So. England 8: 41-4.