This type of sex-determining mechanism was first studied in the milk weed bug, Lygaeus turcicus by Wilson and Stevens. Therefore, it is called Lygaeus type. These are two different patterns of sex determination in Lygaeus type.

# (1) Female homogametic XX and male heterogametic XY : The homogametic sex (XX) is female and produces ova all of one type, i.e. having X-chromosome. The male is heterogametic-XY and produces sperm of two types. 50% of which possess X-chromosome and other 50% Y-chromosome. This is simple XX-XY type and is found in man, Drosophila and certain insects.
Example : In Drosophila total number of chromosomes is eight, of which six are autosomes, common to both male and female. The fourth pair is of sex chromosomes. In male this is represented by XY i.e. Karyotype of male Drosophila 6+XY and in female XX i.e. 6+XX. Ova produced by female are all similar possessing 3+X chromosomes, whereas the sperm produced by male are 3+X and 3+Y in equal numbers.

# (2) Female heterogametic and male homogametic : In fowl, other birds and some fishes, certain moths and butterflies, the female sex is heterogametic, with X and Y chromosome often represented by Z and W and laying two types of eggs, one half with X or Z chromosome and the other half with Y or W chromosome. The male sex is homogametic having XX or ZZ chromosomes. It produces sperm all of one type.

XX-XO type or Protenor type : Mc clung in male squash bug (Anasa) observed 10 pairs of chromosomes and an unpaired chromosome. Their females have eleven pairs of chromosomes (22). Thus all the eggs carry a set of eleven chromosomes but the sperm are of the two types: fifty percent with eleven chromosomes and the other fifty percent with ten chromosomes. The accessory chromosome was X-chromosomes. Fertilization of an egg by a sperm carrying eleven chromosomes results in a female, while its fertilization by a sperm with ten chromosomes produces male. It is said to be evolved by the loss of Y-chromosome.

Haploid-diploid mechanism of sex determination : Hymenopterous insects, such as bees, wasps, saw flies, and ants, show a unique phenomenon in which an unfertilized egg develops into a male and a fertilized egg develops into a female. Therefore, the female is diploid (2N), and the male is haploid (N). eggs are formed by meiosis and sperms by mitosis. Fertilization restores the diploid number of chromosomes in the zygote which gives rise to the female. If the egg is not fertilized, it will still develop but into a male. Thus, the sex is determined by the number of chromosomes.
In honeybee, the quality of food determines whether a diploid larva will become a fertile queen or a sterile worker female. A larva fed on royal jelly, a secretion from the mouth of nursing workers, grows into a queen, whereas a larva fed on pollen and nectar grows into a worker bee. Thus, the environment determines fertility or sterility of the bee but it does not alter the genetically determined sex. The sex ratio of the offspring in the hive is controlled by the queen. She lays more fertilized eggs that produce worker females and fewer unfertilized eggs which produce haploid males. The queen mates only once in her life time, keeps a store of sperms in the seminal receptacle, and can control fertilization of eggs by releasing or not releasing sperms.