First a few definitions to lay the groundwork.
Paedogenetic: Producing young while in the immature or larval state.
Thelytokous parthenogenesis: Producing only females from unfertilized eggs.
Arrhenotokous parthenogenesis: Producing only males from unfertilized eggs.
Amphiterotokous parthenogenesis: (I can't actually find a definition for this word... but it is used...) to describe larva that appear as though they should have produced a male but for some reason have failed to and after a delay begin producing (parthenogenetically) females.
Haploid and Diploid: A diploid organism has two copies of each chromosome while a haploid organism has only one copy of each chromosome.
Ovoviviparous: Refers to animals that produce eggs but retain them inside the female body until hatching occurs, so that "live" young are born. Some of the source papers for this article refer to these beetles reproducing viviparously but I can find no evidence of this in anywhere so I am using this more conservative term.
I created the flowchart below to show all of the different possible developmental pathways. It is amazing to think of the amount of morphological data stored in this genome. These 10 chromosomes are capable of producing so many unique phenotypes. Learning what signals are important in determining the appearance of these phenotypes would be quite interesting.
I found the easiest way to think about this is to start with the
triungulin larva. This larva always develops into a cerambycoid larva. From here
we can go the simple path of producing pupa and then an adult female, or the
alternate option to molt and become a reproductive larva. There are three type
of reproductive larva the most common is the thelytokous larva which is
responsible for most reproduction in a population. This larva Ovoviviparously
gives birth to large numbers (up to 20) triungulin larva. The other two options
are to produce a single male curculionid larva that consumes the mother before
pupating into the adult male form or if the male does not develop for some
reason the larva can after a delay begin producing triungulin larva though not
usually in the numbers seen in the thelytokous larva form.Allan Charles Scott published a paper in 1938 titled "Paedogenesis in the Coleoptera" were he describes his discovery that the fate of larva are decided very early on. He was able to discern significant differences in the ovaries of cerambycoid larva allowing him to identify those that would pupate into adult females, thelytokous larva or one of the other two classes of reproductive larva. These last two classes of larva can not be told apart until much later and it may be that arrhenotokous larva can develop into amphiterotokous larva under the right conditions. The traits that Scott looked at were the number of lobes in the developing ovary and the size of the egg cells.
On the left we see one ovary of the thelytokous larva, in the
middle we have the adult female ovaries, and on the right we have the ovaries
of the Arrhenotokous larva. (Figure from Scott,
1938)Scott's 1938 paper also includes a drawing of all 5 reproductive forms of Micromalthus debilis which I have reproduced and explained below.
Adult Reproductive Forms
D - Adult Females: Are diploid and have 20 chromosomes. They can lay eggs that are presumably fertilized by males. These eggs are believed to develop into triungulin larva capable of any of the various developmental paths. Which development path is followed is most likely controlled by the environmental conditions. Whether these females are capable of laying unfertilized eggs that will develop is unclear from the research that I have found. It is also interesting to note that Scott who has done the most thorough examinations of this species found never more than two eggs close to maturity in females (in contrast the Thelytokous larva can produce broods of up 20 larva). This is particularly odd if we assume that this stage is the primary dispersal stage.
G - Adult Males: Are haploid organisms and have only 10 chromosomes. These males can presumably fertilize adult females they do definitely produce sperm though meiosis does not progress in the normal fashion (only 2 spermatozoa are produced from each primary spermatocyte.
Paedogenetic Forms Reproducing Parthenogenetically
B - Thelytokous Paedogenetic Female: This form of the beetle is the most prolific. Via parthenogenesis and ovoviviparity it produces broods of the triungulin larvae. Like other triungulin larva in this species they are capable of following any of the developmental pathways.
E - Arrhenotokous Paedogenetic Female: This form matures a number of eggs but only one of these is shed. This egg adheres to the mother after being shed and hatches after a very short time. When it hatches the male curculionoid larva begins devouring the mother immediately. This is the only source for larva that will develop into the haploid male.
C - Amphiterotokous Paedogenetic Female: this form has proven to be essentially a male producer (Arrhenotokous Paedogenetic Female) in which the development of the male is arrested and female-producing eggs develop secondarily.
Other stages
A - Cerambycoid Larva
F - Pupa
If there are any obvious or not so obvious mistakes please feel free to let me know.
Source Papers
Pollock, D. and Normark, B. (2002). The life cycle of Micromalthus debilis)(Coleoptera: Archostemata: Micromalthidae): historical review and evolutionary perspective. Journal of Zoological Systematics & Evolutionary Research 40, 105-112.
Scott, A. (1938). Paedogenesis in the Coleoptera. Zoomorphology 33, 633-653.