Lacewings have some distinctive features of digestion, especially in their larval forms. This post will cover:
- Larval Digestive System
- Larval Predatory Behavior
- Prey Search and Recognition
- Prey Capture and Consumption
- Cleaning and Resting Behavior
Adults and larvae have different modes of feeding. Larvae are predatory, although they will dabble in honey, nectar, or honeydew. Most Chrysopidae larvae have a degree of morphological and functional uniformity in their digestive systems, as well as predatory behavior (although little is known about predatory associations in nature, outside of lab studies). Adults may be predatory or not, depending on the genus, which is accompanied by morphological and functional changes within their digestive systems. It is unknown whether or not this affects foraging behavior as well.
Larval Digestive System
There are a variety of ingestive and digestive properties unique to lacewing larvae. They are generally classified as chewing insects, however they possess a unique ‘sucking-type feeding apparatus’ instead of a mouth. They have sickle shaped jaws (made up of the mandibles and maxillae), which are heavily sclerotized and grooved. These are used to catch pray, as well as to cut them up and ingest them.
The mandibles and maxillae come together at a chitinized fold, which serves as the feeding channel for the larvae. Inside the head, these channels merge, forming the pharynx, which then extends to the esophagus. The jaws are often longer than the head, and are always curved inward. Maxillary glands are also present, as well as a pair of unbranched, tubular saliva glands extending from the back of the head to the base of the mouthparts, connecting to the feeding channel.
As mentioned, no typical mouth opening is necessary. The mouth is mechanically closed by the cephalic integument quickly after larvae emerge from their eggs and begin molting. Therefore, the only way for lacewing larvae to eat is by ingesting food through their jaws. This is accomplished by mechanically shredding prey tissue into small parts, and by injecting secretions from their salivary glands, as well as possibly the maxillary glands.
These secretions include hydrolytic enzymes that partially break down large molecules within nutrients, allowing easier movement through the feeding channel, and easier digestion in the midgut. The enzymes appear potent, however the chemistry of them is largely unstudied. Partially digested liquid is sucked up and pumped into the gut, a system that can be described as “intact without reflux”. There is no destruction of the prey’s cuticle, and the enzymes come from extra-intestinal glands rather than inside the gut.
C. carnea, s. lat. has been discovered to have numerous bacteria living within their gut, which likely help decompose food.
Chrysopidae larvae also possess silk glands. In earlier larval stages, they create a stinky brown fluid that can be used to dissuade predators or as an adherent. Later, this secretes silk, which is used to spin a cocoon to pupate in.
Larval Predatory Behavior
*Feeding nature has mostly only been studied in labs, rather than natural environments. It is also important to note that most research has been conducted on Chrysoperla carnea, s lat, and we should avoid generalizing too much. Chrysoperla sp. are characterized by being more aggressive, quicker, and faster to develop than other Chrysopids.
Larvae search until they discover prey. It is then contacted and recognized, and if it is found suitable, it’s captured by the lacewings jaws and consumed. Then, the mouth parts may be cleaned, and prey search is eventually resumed.
It is notable that lacewing larvae are able to withstand food and water deprivation for a relatively long period.
Search, Contact, and Recognition of Prey
Lacewing larvae need to make physical contact to identify their prey. To do this, they maintain a characteristic search posture, in which they move their heads from side to side with the mouthparts partially open, parallel to the substrate below them. They also occasionally sweep their body from side to side. When they make contact with prey, their search becomes concentrated in the area of collision, with more frequent change in direction, as pictured. This is advantageous against prey that aggregates, such as aphids or thrips.
Most Chrysopid larvae are generally active, especially during the night. It has been observed that the more hungry they are, the more intense their activity becomes. If no prey is present, they will gradually reduce movement, become lethargic, and die.
Larvae are photosensitive, and the direction of light has an effect on how neonate larvae move through a plant. This reaction varies by species. Chrysopa slossonae show negative photoaxis, encouraging movement away from the leaves and branches at the upper part of the host, into the center, towards prey living in the lower sections of the plant. Chrysopa quadripunctata have positive photoaxis, encouraging them to remain near the top of the leaf canopy.
Older larvae who have already fed may have various responses to different prey species, influencing searching. Volatile chemicals may aid in larval searching, such as kairomones from honeydew, Lepidopteran scales, and insect eggs. Prey is recognized chemically via the sensory receptors on the end of the labial palpi and antennae, and is likely aided initially by visual stimuli.
Prey Capture
After prey has been contacted and recognized, the larvae displays another characteristic posture: it opens its jaws wide open, parallel to the substrate and slightly upwards, and directs the antennae and labial palpi laterally. It has a series of movements, where there is a slow approach, a halt, then and attack with a rapid forward thrust of the jaws, and a quick retracting of the head, oftentimes while lifting the prey off the substrate. Immobile prey (such as eggs or pupae) are approached differently. They are examined slowly with the mouthparts, then pierces the cuticle in several places. Usually, one jaw pierces while the other is used to manipulate the prey.
Prey Consumption
Secretions from the feeding channel liquefy the internal contents of the prey, which makes them available for ingestion through said channel. Larvae usually exhaust the contents of small prey, and abandon the carcass, however sometimes they don’t set it down immediately. The amount of time it takes to consume prey varies on the size of the predator, size of the prey, and level of larval hunger. The table below demonstrates the time C. carnea s. lat. takes to eat.
| Instar | Egg consumption time (sec) | Caterpillar consumption time (min) |
| 1 | 185 | 13 |
| 2 | 130 | 8 |
| 3 | 80 | 3 |
Similar species may consume different amounts, although third instar larvae exhibit no interspecific difference in feeding efficiency.
Cleaning and Resting
Often after eating, although not always, the mouthparts are rubbed against eachother, or the substrate below, to clean them.
After consuming a satisfactory amount of food, larvae tend to rest. They become inactive, put their jaws together, and extend their labial palpi and antennae forward, with their foretibae (front legs) parallel to the boxy axis.
Conclusion
That wraps up larval digestion and feeding patterns. The next post will cover all of this for adults, who have some interesting distinctive features as well.