Amino acids are chiral molecules; They exist in left and right handed versions. Only the L- forms of amino acids are used in proteins. Curiously, many insects have high levels of D- forms of amino acids. The silkworm, Bombyx mori, has high levels of D-serine. What does it do?
A group of scientists* studied the location and metabolism of D-serine in silkworm. They found it along with serine racemase (an enzyme that interconverts L- and D-serine) in multiple locations. Injection of D-serine caused an increase in pyruvate, suggesting that D-serine enters the Krebs cycle as a source of energy. They consider that D-serine may be primarily utilized as an energy molecule.
This work recalled some experiments I did as an undergraduate in an insect biochemistry lab. We injected milkweed bugs with radio-labeled D-alanine and found that most of what we injected was converted into carbon dioxide. The project was dropped for a variety of reasons, but I suspected at the time that D-alanine could be an energy molecule. D-amino acids are potentially toxic to some parasitoids and pathogens and may have some role in insect defense. This is speculation on my part, but it is interesting to find people still trying to resolve this question.
*Minoru Tanigawa, Chihiro Suzuki, Kimio Niwano, Rensuke Kanekatsu, Hiroyuki Tanaka, Kihachiro Horiike, Kenji Hamase, Yoko Nagata. Participation of d-serine in the development and reproduction of the silkworm Bombyx mori. Journal of Insect Physiology. Volume 87, April 2016, Pages 20–29.