APHIDID.5i:. 283 



exchange of host-plant. Though the principal host-plant is always 

 the spruce, only one generation belongs exclusively to the spruce. 

 Other species on Scots pine and silver fir are (so far as our present 

 knowledge appears to show) peculiar to themselves. This change- of 

 conifers referred to other than spruce is known as the intermediate 

 conifer (Zwischenkonifere) onhj in so far as the former brings about 

 the transport of the species, through the laying of eggs on the inter- 

 mediate conifer, in order that they may return to the spruce. Tliis 

 changing of the insect from one species of tree to another is a most 

 important point, as it clearly implies that this alternation is for the 

 welfare of the species. This shows that it must have been difficult 

 to discover and ratify by those Continental workers, and even difficult 

 for the student in this country to verify. 



The folloAving is a short account of the alternation of the species 

 on spruce known as C. viridis, as given by Nlisslin : ^ — 



" The 1st generation [foundress) is a parthenogenetic female, wing- 

 less, with three-jointed antennae, confined to the spruce and producing 

 the spruce gall. The 2nd generation {migrans alata) acquires wings 

 after the fourth moult, receives at the same time five-jointed antennai, 

 compound eyes, and three simple eyes. It develops in the gall on the 

 spruce, and flies usually to one of the ' intermediate conifers,' where it 

 lays its eggs on the needles. From these eggs springs the 3rd genera- 

 tion (emigrans), which resembles its ancestor of the 1st generation (the 

 foundress). It remains on the alternate host (the ' intermediate 

 conifer') for hibernation, and lays eggs there in spring. From the 

 eggs emerge either forms resembling the parent (exidans) or the 4th 

 generation (sexupara), which in every respect resembles the generation 

 before the last (its 'grandmotherly generation'), the migrans alata, 

 only it remains smaller, and is less prolific. From the beginning it is 

 independent (freilebend), and after the first moult varies somewhat. 

 It lives and develops on the needles of the ' intermediate conifers,' and 

 flies back to the spruce, where it lays eggs on needles. Out of the 

 eggs come the two difl'erent forms of the 5th generation (sexuales) — the 

 leaner males and the stouter females. Both are rather small, but do not 

 diminish in size, and have four-jointed antennae. The female, after pair- 

 ing, lays in midsummer a single fertilised egg on the spruce, from which 

 emerges the hibernating foundress. The fivefold cycle requires, there- 

 fore, two years and two different coniferous species for its completion. 

 ' Leitfaden der Forstinsektenkunde. 



