In the smallest insects, the appendages on the head can be greatly reduced in size or vestigial as an adaptation. Larvae of Stylops ovinae a Strepsipteran parasite of bees is only a few tenths of a millimeter long. Miniaturization has led to drastic alteration in the appendages of the head. Antennae are absent as well as the antennal musculature. The mandibles are the only functional mouthparts. The Labium is fused to the head and the maxillae are fused to each other to form a plate under the mandibles. The mandibles have a grinding “molar” surface at the base and are “knife shaped” at the tip. At rest, the mandibles are completely enclosed by the head and other mouthparts. The mandibles are suited to pull food into the mouth and grind the larger pieces.
A substantial space saver is elimination or reduction of the muscles in the head. This reduces the area needed for attachment as well as the space occupied by the muscles. Stylops ovinae has eliminated at least 3 pairs of muscles for the antennae, maxillae and labium. Muscle elimination allows more space to be devoted to muscles attached to the mandibles, which are the only moveable mouthpart. The result is a head that barely resembles that of an insect.
*Pieter Knauthe, Rolf G. Beutel, Thomas Hörnschemeyer & Hans Pohl. 2016. Serial block-face scanning electron microscopy sheds new light on the head anatomy of an extremely miniaturized insect larva (Strepsiptera). Arthropod Systematics and Phylogeny. 74(2): 107–126.