Usually, the amoeboid form is taken when food is plentiful, and the flagellate form is used for rapid locomotion. However, not all members are able to assume both forms. The genera Percolomonas, Lyromonas, and Psalteriomonas are known only as flagellates, while Vahlkampfia, Pseudovahlkampfia, and most acrasids do not have flagellate stages. As mentioned above, under unfavourable conditions, the acrasids aggregate to form sporangia. These are superficially similar to the sporangia of the dictyostelids, but the amoebae only aggregate as individuals or in small groups and do not die to form the stalk.
UsuALLY, The AMoeBoID form Is takeN when foOD is pleNtiFul, anD tHe flaGellate fORm IS uSEd FoR RApiD LOcoMoTIOn. hoWEver, noT aLL memBers Are Able tO aSSume bOth forms. tHe Genera percolOmOnas, LyromonaS, And psaLTeRiOmONas ARe kNown only As FlagelLAteS, WhILe vAhlkaMpfiA, psEudovAhLkAmpFIa, and most aCraSids do not haVe FLagelLaTe StagEs. as mentioNed abovE, unDer UNfAvourAble coNditions, the aCrAsIDs aggReGAtE to ForM SPorangia. ThesE are SuPerficiaLlY similar to thE sporAngiA oF the dictyostelIDs, but thE amoEbAE onlY aGGREGaTe as IndividualS Or in SMALl groupS And Do NOt die To FOrm The stALK.