Usually differs from other genera of Milichiinae in the combination of posterior eye margin with notch and/or emargination and posteromedial triangular projection of male T1 into T2. A group of Milichiella species in which neither of these character states is present can be differentiated from other Milichiinae by the following combination of characters: Nearctic or Neotropical distribution, 2 frontal setae, posterior eye margin gradually separating from head margin ventrally, and anepisternum bare. (Brake 2009)
Coloration and vestiture: Head and thorax usually brown, black or grey, sometimes with special colour patterns; lunule usually black, sometimes brown or yellow; basoflagellomere usually black, sometimes basomedially yellow to red; face usually brown or greyish microtomentose, sometimes silvery; palpus usually black, sometimes yellow to red; thorax with mesonotum in all variations between shiny and strongly microtomentose; wing and calypter usually hyaline, sometimes white or brownish; brown spot at apex of vein R1 absent to strong, halter colour often different in males and females, male abdomen usually black in ground colour, rarely yellow; male T2-5 often with silvery microtomentum in various patterns; female T2-5 usually brown or black.
Head: Frons with 2-3 orbital and 2-3 frontal setae; frons in males narrower than in females; posterior eye margin usually but not always with notch and narrow emargination.
Thorax: 1-5 dc, 1-5 prsc, 1-2 pprn, 1 prs to row of prs across mesonotum, usually 2 main keps and some smaller setae.
Wing with R4+5 and M parallel to strongly convergent.
Legs: Tibial organ rarely present.
Male abdomen: Posteromedial triangular projection of T1 into T2 usually but not always present; T2 anterolateral corner often with tuft of setae; T2-5 usually with lateral, less sclerotized crease where tergites are bent ventrally; shape and setation of S5 species-group specific; genitalia usually with 1 seta on subepandrial plate, shape, lateral structure and setation of surstylus species or species-group specific.
Female abdomen: Preabdomen typical for Milichiidae. Shape of S4 and S5 varies and could possibly be species-group specific. However, females are unknown or questionable for most species. Ovipositor long and retractable; T6-8 and S6-7 complete, S8 longitudinally divided, cerci usually paired but fused in Milichiella iberica Carles-Tolrá (Carles-Tolrá 2001) and possibly dorsally fused in some other species (M. bakeri Aldrich, M. mexicana Brake). Spermathecal ducts rolled up into one longish coil or with very tightly wound, spindle-like coil (Brake 2000). (Brake 2009)
Egg: Shape elongate, narrower anteriorly. Chorionic sculpture consists of minute, rounded pits in M. tiefii and M. lasuizae sp. n.; M. lacteipennis with even smaller pits. Pits arranged in more or less distinct hexagonal fields in M. tiefii (Roháček 1995). Micropyle connected by radial ribs with surrounding ring in M. tiefii; radial ribs also visible in M. lacteipennis. (Roháček 1995, Brake 2009)
Larva: (Milichiella tiefii, after Roháček 1995): Slender, elongate, narrower on anterior quarter. Cephaloskeleton with slender, strongly sclerotized mouthhooks, with sharply pointed apex and narrow, relatively acute posteroventral apodeme. Dental sclerite rounded tetragonal, weakly sclerotized. Intermediate sclerite separate from basal sclerite, relatively robust; lateral rods strongly sclerotized and connected by slender, strip-like cross-bridge. Complex labial sclerite in front of cross-bridge composed of posterior convex plate bearing 2 lateral (anteriorly-directed) and 2 small medial projections, and anterior slender, v-shaped sclerite (the latter perhaps homologous with ligulate sclerite). Epistomal sclerite (situated anterodorsally between arms of intermediate sclerite) membraneous, with only margins of 2 small perforations pigmented and thereby visible, even in ventral view. Parastomal bars very slender but dark and heavily sclerotized. Basal sclerite very pale, weakly sclerotized, except for anterior part. Dorsal bridge simple, not separated by incision from rest of basal sclerite. Dorsal cornua slender, posteriorly pointed, essentially smaller than ventral cornua; the latter long and broad but without dorsal apodeme. Ventral cornua joined ventrally by downward curving trough with conspicuous, longitudinal ridges. Cephalic region bilobate, with antenna dorsally near apex of each lobe; facial mask bearing oral ridges. Anterior spiracle with 7 digitiform protuberances arranged in flattened, semirosette shape. Posterior spiracles borne on widely separated protuberances; each spiracle without peritreme and not on single plate, but composed of 3 pale yellowish brown, finger-like projections. Each spiracular projection has its own, slightly curved slit and several, slightly ramifying, fine peristigmatal tufts.
Puparium: Elongate, tapering towards anterior and posterior end. Empty puparium yellow to reddish brown, subshiny to dull, with distinct border between segments. All thoracic segments sculptured by distinctive, mostly transverse ribs; prothoracic (both dorsally and ventrally) and mesothoracic (dorsally only) segments with more conspicuous ribbed sculpture. Anterior spiracles with 6-9 (6 in Milichiella lacteipennis, 7 in M. tiefii and 9 in M. asiatica Brake) spiracular lobes (papillae). Abdominal segments circled by very fine ribs or wrinkles, ventrally each with 1-3 transverse rows of very minute spinulae. Posterior end of puparium more convex and strongly transversely or reticularly (last segment) ribbed. Anus visible. For posterior spiracles see larval description. (Roháček 1995, Brake 2009)
Specimens have been reared from various decaying fruit, vegetables or other vegetation (Milichiella near argentiventris Hendel, M. asiatica, M. circularis Aldrich, M. hendeli Brake var., M. lacteipennis, M. novateutoniae Brake, M. quintargentea Brake, and M. unicolor (de Meijere)), with several species of the argentea-group having been caught or reared from palms (Aracaceae). Most species of the aethiops-group, as well as Milichiella acantha Brake and Milichiella sp. L, have been reared from cacti (Cactaceae), e.g. Cereus giganteus, Echinocactus polycephalus, Lophophora williamsii (peyote), Myrtillocactus geometrizans, Myrtillocactus sp., Opuntia occidentalis, Opuntia (Platyopuntia) sp., Pachycereus sp., and Pachycereus schottii. Milichiella aethiops was also reared from “wild tuber”. At least four species (Milichiella argyrogaster, M. cinerea (Coquillett), M. tiefii, and M. urbana Malloch) develop in rotting bark or wood (beech, lime, poplar), which probably needs to be infested first by bark beetles. For details on the biology of Milichiella tiefii, see Roháček (1995).
Various species have been observed on or near plants, e.g. Pandanus (Pandanaceae) (Milichiella aldabrae Brake), Murraya exotica (Rutaceae) (M. argenteocincta Johnson), Croton sp. (Euphorbiaceae) (Milichiella sp. B), cotton (Malvaceae) (M. zaiziksensis Brake var.), baobab (Malvaceae) (M. unicolor), yucca (Agavaceae) (M. zaiziksensis), Cassia leaf (Fabaceae) (M. nudiventris Becker), Shorea macrophylla and S. johorensis (Dipterocarpaceae) (M. bruneiensis Brake), and pawpaw (Asimina, Annonaceae) (M. lacteipennis). Some specimens were observed on flowers, possibly pollinating, e.g. Euphorbia regis-jubae (Euphorbiaceae) and Schinus mollis (Anacardiaceae) (M. bimaculata Becker, M. lacteipennis), Ipomoea tiliacea (Convolvulaceae) (M. cingulata), Lactuca scariola var. integrata (Asteraceae) (M. arcuata), Tamarix gallica L. (Tamaricaceae) and Lonicera (Caprifoliaceae) (M. hendeli), Donnellsmithia hintonii, Asclepias, Baccharis glutinosa, Ochradenus baccatus (Resedaceae), Cocos nucifera (Arecaceae) and Polyscias scutellaria (Araliaceae) (M. lacteipennis), Thapsia villosa (Apiaceae) (M. iberica).
Some species were collected on or near mushrooms or mushroom compost, e.g. Milichiella aldabrae, M. arcuata (Loew) var., and Milichiella lucidula Becker.
Milichiella argenteocincta was found in dirt near an Atta cephalotes nest and reared from excrement in a toucan's nest. Milichiella longirostris Brake and ?Milichiella arcuata var. emerged from the detritus of a gopher tortoise burrow and M. arcuata has been reared from a chicken pen. Other species have been collected or reared from manure and dung of various herbivores or omnivores (Milichiella asiatica, M. lacteipennis, and M. quintargentea Brake), and two species were collected on carrion (M. arcuata, M. lacteipennis). Milichiella cavernae Brake, M. rugosistyla Brake and M. weejasperensis Brake were collected in caves, with the first being reared from bat dung.
Milichiella arcuata is attracted to [E]-2-hexenal and Milichiella sp. prob. lacteipennis to Podisius maculiventris pheromone. Milichiella lacteipennis was observed among aphids on cotton and feeding on bugs, usually Pentatomidae, caught by spiders (Nezara viridula, Nephila inaurata).
Swarming behaviour has been observed in males of M. bermaguiensis Brake, M. circularis, M. frontalis (Becker), M. hendeli, M. lacteipennis, M. longiseta Hardy & Delfinado, M. nigeriae nigeriae (Duda), M. sumptuosa de Meijere, M. tiefii, M. unicolor and M. zaiziksensis. These swarms were sometimes located above or close to the breeding substrate. (Brake 2009)